Besides today, that is?
The most important day to practice is the day of your lesson, immediately following the lesson. Playing for even a few minutes is enough. This simple act will affect your practice results for the rest of the week.
There are two reasons for this:
1) Reviewing what happened in the lesson while it is still fresh will reinforce the improvements and tweaks made during your lesson, and greatly increase the possibility that you'll remember and implement the suggestions your teacher made throughout the week. If you wait until three days later, you may not even remember what the suggestions were about even if the teacher's comments were written down, thereby making the lesson useless.
2) Practicing when you come home after the lesson creates a sense of momentum that will carry you through the rest of the week. When you make the effort to practice a little right when you get home from the lesson, you'll feel good about yourself and your results, and you'll be that much more motivated to get back to it then next day, and the day after that, and so on. This creates even more momentum, as you and your teacher will be thrilled by the improvement you'll exhibit and you'll continue your achievement into the next week and the next.
If you can't practice immediately after your lesson, at least practice the day after the lesson. Try this and you'll be amazed at how much more you will enjoy your practice time.