A weekend, technically, is just Saturday and Sunday - but it is often simply used to mean the period between when a person is working on one week and when they start working on the next. This is the colloquial usage in the following phrases:
It's 5:30 (PM) on Friday. My phone is going off, I'm not going to respond to emails, and I'll be back on Monday. The weekend for me starts now!.
Since Monday is a Bank Holiday(1), have you got anything planned for the long weekend?
I'm going to take Friday off work and take the wife on a romantic weekend to Paris.
For holidays which include either the Monday immediately following a weekend or the Friday before (or both), the holiday can be colloquially described a long weekend.
(1) A Bank Holiday is a UK National Holiday.