There are couple of request similar to this, I'll post another one to draw more attention.
The needs for User (when fill in the form) and admin (when receiving Submission in PDF format) to see the DAY is important, instead of just seeing date.
There are a couple suggestion I can think of :
1. Provide date format with DAY in the DATE field itself. Currently available format is just with dd/mm/yyyy (in any order). Provide format with ddd or ddddd like we have in Excel might be easy fix for this issue.
2. Enable DATE field to be used in reference. This more like a workaround, we still need to make reference data for e.g. the whole year to get the DAY for each date.