Here is my personal impression of the different days at Union Square:
MondayGood day to practice because it is not often there is a crowd. The band starts at every hourly (9pm, 10pm) and they usually start off with some slower music, giving you a chance to practice your moves you learn in class or to try other new moves.
TuesdayTuesday is usually more packed because it is Beginner Night. Therefore, you would get a lot of students coming down from the different schools. A better night for social dancing than a practice session. It also gives you a good chance to dance with people from other school.
WednesdayLike Monday, it is usually a little quieter. However, the JJSalsaRengue Rueda (casino salsa) students do come down after their rueda class to practice. Rueda usually takes up a bit more space so leaders would need to keep their eyes open to protect their followers. However there is usually enough space to get some practice done as well if you wish. It's also a good way to chill mid-week during a hectic work schedule.
ThursdayThursday is usually a quieter day. Good for practicing as well. There is the occasional rueda group but usually, you would have the whole Union Square to yourself. Gives you lots of practice of dancing on a nearly-empty dance floor.
FridayFriday is jammed packed because it's LADIES night as well as the end of week. You get a wide range of dancers at different levels, as well as styles. This is a night that should be kept for social dancing because most Friday nights would have dancers dancing shoulder to shoulder. Like Tuesday, leaders would have to keep a close eye on the surroundings so as to ensure no injuries to their followers.
SaturdayLike Friday, Saturday is the night everyone comes out to party. Union Square is usually jammed packed (only time it was slightly emptier was during World Cup Season, but that comes once in every four years). Same thing, leaders please protect your followers.