Weekend is singular and weekends is plural. So, if I am talking about this coming weekend or last weekend or now, this weekend, it is singular. If I am talking about a course of study that I did five weekends in a row, or that I will be working for three weekends, or in any other way talking about more than one weekend I say 'weekends'.

Hello Anna: Weekend refers to one weekend. You might say, "I will see you this weekend." That means a specific, and single weekend. . . However, if a friend asks if you are ever free to meet, you might say; "I am sometimes free on the weekends." That is a reference to more than just one weekend. You can be free on many weekends. Hope that helps. --Warm Regards, Bruce