Other forms: weeknights
If it's evening, and the next day isn't Saturday or Sunday, it's a weeknight. Don't plan that sleepover party for a Tuesday — it's a weeknight, and you've got to get up early the next morning for school.
During the regular work week, most people tend to go to bed before it gets too late. On weeknights, you're less likely to go to a movie that starts at 9:00 or invite your friends over for a late night dance party. Whether you've got work or school in the morning, weeknights are often all about being responsible. Your favorite restaurant might even close early on weeknights, while staying open until midnight on the weekend.