To add to [Irin's answer](https://webapps.stackexchange.com/a/115095/142901), here is a formula that you can use for general situations.

    =match(B$1,{"Sat","Sun"},0)

See [here](https://support.google.com/docs/answer/3093378) for documentation on `match`.

`{}` is the format to concatenate arrays. The operator `;` concatenates arrays vertically, ie. stacking rows; while `,` concatenates horizontally, ie. stacking columns. In this case, we are just creating a single row *ad hoc* comprising of the list of strings you would like to match to.

Besides `match`, you can also use `find` and obviously a combination of `exact` and `and`.