It's a bit tricky but you can do it.

1. Add new column called "DAY of the week"

2. Select  RichText(html) as type

Now, pick one row you want to edit. On the Day of the week field, switch to "Source" mode. You would want to enter the html directly. Click "Source" button left upper-corner of Rich Text Editor.

Then enter following code (Obviously, you need to pick one line from following.)

<INPUT type=hidden name=Day0>Sunday
<INPUT type=hidden name=Day1>Monday
<INPUT type=hidden name=Day2>Tuesday
<INPUT type=hidden name=Day3>Wednesday
<INPUT type=hidden name=Day4>Thursday
<INPUT type=hidden name=Day5>Friday
<INPUT type=hidden name=Day6>Saturday

On the list, users don't see the tag because it's hidden tag. However, the program reads it and use it to make orders. The text on "Day of the week" doesn't have any sorts of order information so we need to give it to them as Day0, Day1, Day 2 and such. If you want to make it start from Monday, Just change the number on hidden tag.