Answer:
That's just a fun way to say, "I want you every day of the week" as there are six other days in the week... Besides for Sunday.
While most would state the above in their reason for saying it, the more adventurous might concede that the 'boring' conventional "missionary position" can be saved for Sunday (given that it's been long considered a 'respectable' day, normally one of worship) but that there's LOTS of other 'less respectable' positions and sexual acts that can be done during the other six days.