The liturgical season of Lent begins with Ash Wednesday and ends with the start of the celebration of the Mass of the Lord’s Supper on the evening of Holy Thursday.
The only law the Church imposes on us during Lent, at least in this country, are the two laws of fasting and abstinence: abstinence from meat on Ash Wednesday and the Fridays of Lent for anyone 14 or older, and fasting on both Ash Wednesday and Good Friday for anyone 18-59. Since Good Friday is during the Triduum rather than Lent, and since the Church still imposes both of those laws on Good Friday, I do the same with any Lenten devotion I take up by keeping it through at least Good Friday as well.
That being said, there is no requirement that I do so. Lenten penances are private devotions that are not regulated by Church law. There is no requirement that you must “give up” something for Lent, or “go the extra mile” for Lent. There is no requirement that if you do something special for Lent, you must do it for any particular period of time or in any particular manner. You can give up something for all days during Lent including Sundays, or you can leave out Sundays or any other day (such as the celebration of a birthday or anniversary). You can plan to volunteer time feeding the poor at a homeless shelter for just the first, third, and fifth Thursdays of Lent, or spend time visiting the elderly at a retirement home every weekend, or spend every morning praying Morning Prayer from the Divine Office. You could even combine two or more things.
The most specific thing you can find in the Code of Canon Law on the matter of personal penance is the first sentence of canon 1249, which says, “The divine law binds all the Christian faithful to do penance each in his or her own way.” That’s it. There are no rules to question when it comes to private penance, because the person who does the penance is the one who sets the rules. Ask yourself, not others, what you promised at the start of Lent. Then do it.
During Lent we should be focusing on prayer, fasting, and alms giving when we express charity to others, in order to put us in the right spiritual disposition for a fruitful celebration of the Easter season. Turning it into a legalistic exercise doesn’t seem to me like it would be conducive to any kind of spiritual disposition likely to bear good fruit, especially in light of the fact that there’s no law to get legalistic about in the first place.
The bottom line is, yes to being charitable and no to being scrupulous. If you have a legitimately charitable reason for ending your Lenten penance with the start of the Mass of the Lord’s Supper instead of with your celebration of Easter, then do so. If the reason is selfish rather than charitable, then don’t. If you aren’t sure whether your reasoning is charitable or selfish, ask your priest or spiritual director for their advice and then follow that advice.