What is the most important day to practice? Well everyday of course!
If you are taking lessons or attending classes, however, there is a best day. It is either the day of(afterwards) or the day after your lesson. When you learn something new you need to reinforce the concepts and muscle actions ASAP. If you wait until the next day and then you get an unexpected invite, then the next day the kids need you, the next day you already had plans….etc., by now half a week or more has passed. It is easy to forget exactly what you were supposed to be focusing on. If you miss a day after you already practiced your new stuff you have a better chance of remembering everything.
So that said jump on it as soon as you can!