The return value of the expression EEE is language dependant. For only the english language this works fine. But when you use another language such as dutch you can use something like the following expression:
formatDateTime( $Date, 'EEE') = 'Sat' or
formatDateTime( $Date, 'EEE') = 'Za'
You need to use:
formatDateTime($Date,'EEE') = 'Sat'
i.e. you need three E's to get a three letter representation of a day.
Edit based on the comments: the returned representation of a day is based on your language settings. You can detect what value is returned by this function by putting it in a showMessage activity. It doesn't matter if you use 'EEE' or 'E'. Both expressions result in 'Sat' when your language settings are English.