You are missing some quotes ie [$Date = '[%BeginOfCurrentDay%]'].
It looks like you are mixing XPath and Microflow expressions: what you wrote is a comparison of a date variable to a token, not an assignment.
To assign a date to a variable, use a create variable action (or a change object action). Then, you can click on generate, select token and select the correct token from the drop down list. Alternatively, you can set a specific date or assign a value from an object using $Object/DateTimeAttribute.