HDOs are supposed to be treated the same as Sundays AFAIK. 

Then the question is what counts as a HDO. For example, today is Corpus Christi. I read something a few days ago, I can't remember where now, that it hasn't been a universal HDO for a long time. Then different countries have different ones observed. This month there's also Sacred Heart of Jesus on Friday the 16th.

The difference is between precept and counsel. Precept is what we must do as Catholics while counsel is about what we should do. Even if the Church doesn't clearly say in 2023 that today is a HDO, I figure if you can, you should. The extra attention to avoid unnecessary shopping helps to sanctify the day, even if it means simply packing a thermos instead of buying that coffee. Of course, it's not so easy for people who can't take time off work midweek especially if their job is about buying and selling. 

Thanks for the answers. I was treating it like a Sunday just to be on the safe side.

The following is from Question I, Article 7, in Prümmer (1956):

3. Which days are to be kept holy?

By common law and according to present discipline all Sundays and the following ten feasts are days of obligation: the Nativity of Our Lord, the Circuмcision, the Epiphany, the Ascension, Corpus Christi, the Immaculate Conception, the Assumption of our Blessed Lady, the feast of St. Joseph her Spouse, the feast of SS. Peter and Paul, the feast of All Saints (c. 1247). By local law some of these feasts are excluded and others have been added, v.g. the feast of St. James in Spain.

As to the definition of servile work,

§ 3. Prohibition of Servile Work 

426. Distinctions. There are four types of work : 
1. Servile work is that which a) requires mainly bodily activity, b) has as its immediate purpose the welfare of the body, c ) was formerly done by slaves; e.g. farm work such as digging or ploughing, mechanical work like sewing or making shoes. N.B. The character of servile work is not determined by the worker’s intention or by the fatigue involved, or by the fact that wages are received, etc., but solely by the nature of the work itself which remains servile even if done out of charity or for the sake of recreation. 
2. Cultural work is that which a ) is the product chiefly of the mental faculties, b) is immediately directed towards the development of the mind, c) used to be performed by persons who were not slaves, such as reading, writing, singing, playing the organ. These acts remain cultural even if energy is lost in their performance and wages received. 
3. Ordinary ( natural ) work is that which is done indiscriminately by all classes and is chiefly intended for the daily sustenance of the body, such as eating, hunting, travelling, cooking. 
4. Judicial and commercial work is that which is transacted in the courts or in the course of public trading, such as sitting in court, defending criminals, buying, selling, leasing, etc. Note. There are forms of work whose exact nature remains in doubt. In order to solve such doubts one should be guided by the common opinion of men. Thus, for example, rowing is servile work, but common opinion regards it as lawful on Sundays if done for the sake of recreation. 

427. Principle. All servile, judicial and commercial work is forbidden on Sundays and holy days, but cultural and ordinary work is allowed (c. 1248). Any form of servile, judicial or commercial work prevents man from giving sufficient attention to the worship of God, since it absorbs the attention of the mind and tires his body. Other forms of work do not have the same effect. A more lenient attitude towards commercial work is at present in existence, since markets are allowed for the sale of small articles such as flowers or fruit, and private contracts of buying or selling are also permitted. The prohibition of servile and judicial work is grave but allows of parvity of matter. It is thought that servile work lasting for more than two hours (either continuously or with intervals) without any excusing cause constitutes grave matter and is therefore grievously sinful. But if the work is light in character rather than servile, a space of three hours is considered necessary before grave matter exists. 

428. Causes which excuse from this precept can be reduced to three types : 
1. personal need or that of another ; 
2. legitimate custom ; 
3. legitimate dispensation. Personal need or that of another sometimes excuses from this precept, as, for example, farmers during harvest-time, the poor, domestic servants, workers responsible for the maintenance of machines in factories. Some necessity' is thought to exist if there is danger of sinning as the result of idleness. Custom in certain places excuses hair-dressers, drivers of public vehicles, hunters, fishers, those who sell small articles. A dispensation in this law may be granted by the Holy See and also in particular cases by bishops, religious prelates, parish priests for their own parishioners (c. 1245). A confessor has no power to dispense in this matter but in doubtful cases he may interpret the law and allow his penitents to undertake necessary' work.

Similarly, McHugh and Callan (1958), Question III, Article 1:

2579. Servile Works.— The prohibitory part of the precept is concerned with servile works, that is, labor of a kind that tends to make one unfit for devotion or that shows disrespect for the sacredness of the day, even though the labor be done gratis, or for recreation, or out of devotion. Hence, the law forbids: 
(a) works given to the service of the devil, that is, sins that deprive one of holiness, such as riotous recreations, gambling, drunkenness, reading improper matter, and attendance at evil movie performances. But these works are opposed to the end, not to the text, of the law; and hence the circuмstance of time aggravates their malice but does not give them a new species (see 2314); 
(b) works given to the service of the body (servile works properly so called) or to the service of external goods (forensic and commercial works). Servile works in the strict sense cause bodily fatigue and are taken up with material things, and hence they distract the mind from religious thoughts. Such are manual labors (e.g., plowing, digging, housecleaning) and mechanical or industrial labors (e.g., printing, building, plastering, shoemaking). Forensic and commercial labors (e.g., arguing in court, auctioneering) are also of a very worldly kind and unsuitable for the quiet and recollection of Sundays and holydays. 
2580. The prohibitory part of the Sunday precept does not affect works which are no impediment to devotion and which cast no dishonor on the day. Such are: 
(a) works devoted immediately to the service of God. The purpose of the law is to allow leisure for these works, and hence manifestly their performance is not forbidden. Such works are saying Mass, preaching, administering the Sacraments, singing in church, and visiting the poor and sick (John, vii. 23; Matt., xii. 5). But works that are only remotely related to divine worship (e.g., cleaning the church, painting the altar, repairing the vestments, decorating the shrines) should not be done on Sunday without necessity; 
(b) works devoted to the service of the mind (liberal works). These works are of a more elevated kind, do not require great bodily exertion, and are not looked upon as unsuitable to the Sabbath. Such are intellectual works (e.g., teaching, reading, writing, studying), artistic works (e.g., playing the organ, singing, drawing, painting a picture, embroidering), and works of recreation (moderate sports or diversions such as baseball, tennis, and chess). 
2581. Other Kinds of Works and Sunday Observance.—
(a) Common works are those that stand between the liberal and the servile, since they are exercised equally by mind and body, such as walking, riding, hunting, and fishing that is not very laborious. These are lawful. 
(b) Doubtful works are those that are now non-servile, now servile, according to the manner in which they are conducted, such as the work of painters, sculptors, typists, seamstresses, and photographers. Thus, it is a liberal work to paint a portrait, a servile work to paint the walls of a house. In settling the character of various kinds of work, one must be guided by the prudent opinion of one’s locality, and in case of doubt and need must seek a dispensation. (For a history of the theology of servile works see Franz X. Pettirsch, S.J., “A Theology of Sunday Rest,” _Theology Digest_, Vol. VI, no. 2, Spring 1958, pp. 114 ff.; for a survey of modern studies on the problem see _Proceedings of the Catholic Theological Society of America_, 1957). 
2582. Is it lawful without necessity to hire the servile work of non-Catholics on Sunday, if these persons are not thereby impeded from the natural duty of worshipping God and no scandal is given? 
(a) If the non-Catholics are infidels and not bound by church laws, this is lawful. The same would be true of those who lack the use of reason (see 427 sqq.). 
(b) If the non-Catholics are heretics, it is not lawful in the case given to make them work on Sunday.
2583. 
[...]
(b) Prohibitive Part.— Grave matter is labor that is notable on account of its quality (e.g., forensic proceedings even for a brief space on Sunday would be a serious distraction and scandal), or its quantity (e.g., two and a half hours given to very exhausting manual work, such as digging a ditch, three hours given to less arduous labor, such as sowing). He who commands ten laborers to work an hour each on Sunday cooperates in ten venial sins (see 219), but he may be guilty of mortal sin on account of scandal.

Don't confuse manual labour with servile work, especially if the manual labour is connected to one's hobby (recreation) or health.

The sabbath is made for man, not man for the sabbath.

All three things I listed there are indeed servile work, in the case of the car wash, forcing someone else to do that work.  Just running the car through a fully automated facility wouldn't be, but I refer to car washes where they have crew there spraying pre-treatment, brushing the car off, and so on, before you drive onto the conveyer belt.

As a practical matter, being a full-time caregiver to my invalid mother often forces me to do things on Sunday I'd rather not do --- at this time I am even unable to get to Mass --- but that falls under necessary servile work, not unnecessary.   I keep a lot of irons in the fire.