I can't find much information on this.
I know that the Eucharist fast started at midnight and that mass couldn't start after 1 PM. But what about the earliest time a mass could be said?
What year did this change occur?
I know that Pope Pius XII made quite a few changes with the half modernist 1955 Holy Week, but I think I read somewhere that the mass times were changed before 1955.

I can't find much information on this.
I know that the Eucharist fast started at midnight and that mass couldn't start after 1 PM. But what about the earliest time a mass could be said?
What year did this change occur?
I know that Pope Pius XII made quite a few changes with the half modernist 1955 Holy Week, but I think I read somewhere that the mass times were changed before 1955.

Don’t blame the holy week rites on Pius XII. He only had direct correlation to the changing of the dates of certain Holy Week masses, you have to remember he was bedridden for the last years of his life. 

But what about the earliest time a mass could be said?

Presumably anytime after midnight.  So, for Christmas Mass, +Williamson once told me that as long as the Offertory began after Midnight, it counted as a Mass on Christmas Day.

Off topic, but...If the Offertory has to be after midnight for the mass to be a Christmas mass, then this is another proof that the consecration is not the ONLY factor to consider, when determining the completeness/legality/morality of a mass.  
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Not necessarily responding to you, Ladislaus, just a general comment related to last weeks’ thread.

I'm not sure if you can infer this from that guideline.  If you recall, that was also where traditionally the "Mass of the Faithful" began.  When moral theologians were dealing with when you had be be present in order to (minimally) fulfill your obligation, they used the Offertory, since it's the Mass of the Faithful.  Now, the Novus Ordo switched it to the Gospel.

I can't find much information on this.
I know that the Eucharist fast started at midnight and that mass couldn't start after 1 PM. But what about the earliest time a mass could be said?
What year did this change occur?
I know that Pope Pius XII made quite a few changes with the half modernist 1955 Holy Week, but I think I read somewhere that the mass times were changed before 1955.

I have been under the impression for a long time, and still am, that the earliest time Holy Mass can begin corresponds with the break of dawn or close to that moment of the rising of the sun. And, because the birth of Christ as well as His Holy Resurrection happened at midnight, Holy Mass is allowed at that time, too, (maybe almost required?) for liturgy and creation to blend beautifully.  The priest facing east while offering the Holy Sacrifice is another example of the Church's blending liturgy with the life of Christ.  

I have been under the impression for a long time, and still am, that the earliest time Holy Mass can begin corresponds with the break of dawn or close to that moment of the rising of the sun. And, because the birth of Christ as well as His Holy Resurrection happened at midnight, Holy Mass is allowed at that time, too, (maybe almost required?) for liturgy and creation to blend beautifully.  The priest facing east while offering the Holy Sacrifice is another example of the Church's blending liturgy with the life of Christ.  

Well, yes, there was the ancient symbolism of having the Mass, perhaps even the Consecration, coincide with the rising of the sun ... but the Church has long reckoned the day to begin at midnight and so Mass for that day can be celebrated any time after that.  It was common before Vatican II for parishes to offer a 5:00 AM Daily Mass, well before sunrise during most of the year, to accommodate the faithful who have to get to work early.  Now, there is an ancient custom of the following day beginning after sunset of the previous day ... which is why the Vespers "anticipate" the following day's feast (or do they?).  That's the argument for the Saturday Vigil practice in the Novus Ordo.  By itself, it could be OK, since when the one day ends and the other begins could be somewhat arbitrary and the most ancient tradition is in fact that it begins after sunset.  But the Novus Ordo is inconsistent by having a 4:00 PM Saturday Vigil Mass followed by a 5:00 PM Sunday afternoon Mass ... often at the same parish.  Either the Lord's day begins at 4PM or it does not.  If it does, then 5:00 PM Sunday Mass is too late to count for Sunday.

By the way, I very much agree with Pius XII changing the time of both the Holy Thursday Mass and the Easter Vigil.  There's something very wrong with having the Easter Vigil on Saturday morning.  Our Lord rose on the Third Day, not on the Second.  And Our Lord did offer the Holy Thursday Mass in the evening.

It seems like I once read in a pre-Vatican II moral theology textbook that Mass could begin anywhere between 1 hour before sunrise and 1 hour after noon, but I'm going off of memory on that.

I don't know what the rules were apart from midnigt fast but in our parish in St Dublin, Ireland, in the 1940s and 1950s there was 7,8,9,10,11 and 12 Mass. Most inrteresting was that Holy Communion was not served at 11 or 12 Mass. I recall my dad liked a pint on Saturday night and nearly always went to 12 Mass. Indeed a lot of Irishmen liked a pint or two on Saturday night so that could explain why no Communion was given on those late Masses. The most popular Mass was the 8-o-clock one. In those days I served as an altarboy so got to know these things.

During the 1950s, Pope Pius XII requested that Fr Bea, a Jesuit of course, revise the translation of the Psalter, the words of Psalm 26 that contained “I will wash my hands among the innocent” the one used for centuries in the part of the Offertory Rite of the Mass where the priest washes his hands. The last translation of this Psalm was done at the Council of Trent, yet Pope Pius XII wanted an updated translation. Fr Bea, made a cardinal by Pope John XXIII in 1959, was a liberal who also championed the modernist religious liberty at Vatican II against the religious tolerance of the traditionalist Cardinal Ottaviani causing division and rupture at the Council.

https://www.jstor.org/stable/43719062?seq=1#page_scan_tab_contents