Both sentences are correct.

There is a slight difference in meaning. In the first one, the person will play freesbee during future weekends while the second one is about a custom that include past weekends too.

In the second sentence, si can be replaced by quand, lorsque, or chaque fois que without significantly changing the meaning while in the first one, only si is acceptable.

As Jiliagre says, both sentence are grammatical with distinct meaning. In the second sentence, quand would be more common than si if it’s meant as a regular thing you do every weekend. If it’s meant to cover only the coming weekend, the formulation would be in the future with ce weekend or le weekend prochain.