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.

`{}` and the concatenation operations are convenient tools. It's nevertheless better for you have a place where you store all possible strings and use that array instead of the ad hoc `{"Sat","Sun"}`. You can then use data validation to ensure you are inputting the right string every time.

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

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For completeness, I shall also include a quick review on custom formula in Conditional Formatting.

Recall that with any custom formula in Conditional Formatting, you are always inputting a range and a formula relative to a single cell. The custom formula should always output a boolean value. The spreadsheet will automatically iterate the cell number across your range.

Suppose your range is `A1:C3` and your formula depends on `B3`.  That means, when it comes to formatting, all cells in `A1:C3` should use the value in the cell 1 column to the right and 2 row below as the input of your formula. For example, if your formula is `isblank(B3)`. Then `C3` will depend on whether `D5` is blank. 

On top of that, `$` operator works as usual.