It's often used to refer to religious festivals like Christmas (Nadolig), Easter (Pasg), or Halloween (Nos Galan Gaeaf), but can also refer to other celebrations like cultural festivals or public holidays.
Here are some examples:
* Gwyl y Nadolig: Christmas
* Gwyl Pasg: Easter
* Gwyl Dewi Sant: Saint David's Day
* Gwyl y Gwanwyn: Spring Festival
* Gwyl yr Haf: Summer Festival
* Gwyl y Degwm: Tithe Festival