Venial sin is entirely up to you if you confess it in Confession.
That being said; you are absolved of your venial sins every time you go to Mass. The priest gives absolution just before communion IN THE TRADITIONAL LATIN MASS. Otherwise people would have to go to confession before every communion.
However, it is good to confess venial sins 1) to help us overcome them 2) to receive the graces from confession.
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When you enter the Church, use the Holy Water at the doorway to dip your finger in and make the sign of the Cross, touching the water to your forehead, breast, left and right shoulders. With the intention of contrition for your venial sins, they are forgiven through the power of the Holy Water. If you don't have any Holy Water, you can use a blessed Rosary to do the same thing. Sacramentals have the power to invoke absolution for venial sins, but not for mortal sins -- they require a priest's absolution.
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However, in extreme situations, bringing yourself to perfect contrition can take the place of having a priest hear your confession and absolving your sins, even mortal sins. But be very cautious with this doctrine. Perfect contrition is by far not so easy to achieve!
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In order to practice perfect contrition, it takes some getting used to. In fact, it is something that we ought to deliberately try to evoke in ourselves, and a great time to do it is while you're waiting in line for confession. When Protestants enter a Church and walk past Catholics who are waiting in queue for Penance, they are always taken by the appearance of the penitents. Catholics waiting to confess their sins are generally not looking for someone to make eye contact with. And if they happen to do so, they're likely to avert their gaze so as to continue thinking about what they're going to say to the priest when it's their turn.
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It is a good exercise to have in mind while you're awaiting your turn, to imagine that you're in a wilderness area, and dying of some kind of injury, facing the prospect of going to your particular judgment without having been to Confession lately. Therefore, you have the chance now to rehearse what you would then face, having to evoke in your soul perfect contrition for your sins. Try by testing yourself in any way you can imagine, the more challenging the better, since there would be no preparation too effective for training purposes in anticipation of the real thing. If you're ever caught in a bear trap bleeding to death, you won't have much time to start learning then. Or if you're facing an imminent plane crash, that's not the time to start working through the challenges. How much time did the inhabitants of Heroshima or Nagasaki have (the two largest Catholic communities in 1946 Japan). Or consider submariners in a sinking sub -- at the moment of implosion, it's like a bomb going off, but in reverse. It "blows in" not blows up. About 1/10th of a second.
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Sometimes people die in auto accidents. Other times they survive. I happened to be a witness to a head-on collision. The cars were going about 25 mph, but remember, that's 50 mph effectively. The elderly woman who caused the crash was a bit shaken up but not seriously hurt. I spoke to her trying to comfort her. She was appreciative. I asked her if she would like to pray with me, and she said, "Yes." So I began, "Hail Mary, full of grace, the Lord is..." she interrupted me: "Oh, no, I'm not Catholic! We don't pray to Mary." I told her that if she would try to offer to God her sufferings as a sacrifice to God for the sins she has committed, that it would be a great benefit for her forgiveness from God, and she replied, "Oh, I've never heard of that! Are you saying something like 'works' to make up for sins? Works are no good for that..." It was really hopeless. It taught me that at the moment of suffering or tragedy there is no time to start learning perfect contrition! It's something you need to have been well prepared for in advance, so you can simply go into high gear on a moment's notice!
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Anne had been in a 1967 Chevy Impala, and had had a split second's time of inattention, looking for something in her car, when she had drifted about three feet over the centerline of the street, to collide with an oncoming smaller car. The young couple inside were critically injured, as their car had reversed direction instantly.
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BTW, she asked me to walk down the street 150 yards to the next intersection and turn left, where her house was on the corner. "Tell my daughter, Sonya, that her mother, Anne, just got into an accident and needs her help." So I did. I went quickly to knock on the door, and ask "Are you Sonya?" (Hesitantly, "yes...") "Your mother, Anne, just got in a car wreck and asked me to tell you she wants your help." Sonya dropped everything and went immediately to help her mother, and I figured it was time for me to get out of there so they don't sue me for making a mistake or whatever. Long story short, I went by the house 2 months later and it was all boarded up, abandoned, weeds starting to grow, spider webs, peeling paint, etc. It was weird to see the peeling paint, but it was none the less real. 6 months later it was for sale. After a couple of weeks, it was renovated and sold again. Then someone moved in and they didn't know anything about Anne and Sonya's tragedy, nor did they care.
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Names have been changed to protect the innocent.
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