Just had a quick look at my best days in order of percentage profit:

Tuesday
Thursday
Friday
Sunday
Monday
Saturday
Wednesday

So Tuesday is my best day and Wednesday is my worst. Sunday is in the middle.

If you find Sundays don't work for you then just leave them out until the results tell you otherwise.

It could also be that the professional punters put more effort into a Saturday and then on a Sunday they tend to get a few things wrong due to overworking on Saturday which can affect the odds.

I also like the jockey theory posted by CP.

I suspect Saturday and Wednesday are lowest for me as professionals would be getting the odds right on these days and my edge is smaller.