To add to Irin's answer, here is a formula that you can use for general situations.
=match(B$1,{"Sat","Sun"},0)
See here for documentation on match.
{} is the format to concatenate arrays. The operator ; concatenates arrays of matching length 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 or.
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 indices 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.