You are better off storing your days as numbers corresponding to the day of the week. In the UK, 1 is Sunday which represents the first day of the week.
Here in the US, 0 represents Sunday (again, the first day of the week.)
This way, when you sort, it will always sort it as 0, 2, 4 for Sunday, Tuesday and Thursday.
Then use the date() function to translate that day into the actual day's name... It is a bit trickier, but a little tough work early on, will make things tons easier later on...
Believe me, I am finishing up a Calendar app I am writing, and have found it to be easer to manipulate later when the days are numbered instead of defined by text.