DAY_OF_WEEK

Returns day of week of the provided date as a string ('Monday', 'Tuesday' etc)

DAY_OF_WEEK (date)

Attribute of the Date type or date constant or calculation producing a date

Text: the day of the week of the date as a name ('Monday', 'Tuesday', etc.)

IF DAY_OF_WEEK (Account.OpeningDate) = 'Wednesday' THEN ...