Tables are NOT spreadsheets, and do not support calculations; and you should
not store any data which can be calculated.
To display the day, just use two textboxes on your form or report, bound to
the same date field; set the format of one to Short Date (or whatever format
you prefer), and the format of the other to "dddd" to display Saturday or
Tuesday.
If you're interacting with your data in a table datasheet... don't. That's not
their function; use a Form instead.