Help with field please

Help with field please

Within my database I am using a drop-down field to sort the days of the month using the grouping feature. it works very well apart from one thing.

In the drop-down choices I have (for Sept)

01 Sunday
02 Monday
03 Tuesday
04 Wednesday

al the way down to 

30 Monday

Next Month (Oct) the 1st is a Tuesday

So I have to edit the field and change the choices to

01  Tuesday
02  Wednesday
03 Thursday
04 Friday

al the way down to 

31 Thursday

Is there a way to automate the field please

Thanks