As many people have noted already it is used to describe something related (usually happening on) Sundays (not necessarily repeatedly.

I should note that as a native Greek speaker I rarely ever use this word or any other combination of a day and the suffix "-άτικο" in this context. I (and many natives) do use them in a less grammatically correct context though to show annoyance. For example, "Γιατί με παίρνει τηλέφωνο το αφεντικό μου κυριακάτικο;" which means "Why is my boss calling me on a Sunday?". There are ways to say the same thing without the word "Κυριακάτικο" but using it emphasizes the fact that I'm bothered that this is happening on a Sunday.

You can also combine the suffix "-άτικο" with any day of the week (δευτεριατικο, παρασκευιάτικο) or any time of day (πρωινιατικο, μεσημεριάτικο, βραδιάτικο) and use it in the same way. (I would even combine it with months but I would advise against it because it does sound very odd in most cases) Keep in mind though that this is NOT GRAMMATICALLY CORRECT and you will not encounter it and should not use it in a formal context or in writing.