When you have a job, your payday is the day you get paid. You might wait to buy yourself a new outfit until after payday.
Some jobs have a weekly payday — maybe Fridays are payday at the ice cream shop where you work. In other cases, you have to wait two weeks, or even a month, between paydays. You can also use payday to mean a sudden success or influx of money, either earned or won: "That movie brought a huge payday to its producers." Since 1932, there's also been a peanut and nougat-based candy bar called PayDay.