In the Catholic Church, Sundays are usually celebrated according to the season of the liturgical year, such as Ordinary Time.
However, on certain occasions the seasonal Sunday celebration is replaced by a feast.
This doesn't happen often, but it can when the following criteria are met.
To give a few examples, here are a few feasts and solemnities that can be celebrated on a Sunday.
There are others not listed here, but this gives a brief survey of the various feasts in the Church's year that are on fixed days. When that day is on a Sunday, it can replace the typical seasonal celebration.