Obviously I'm going to test, but my gut feelings are:
Tuesday and Wednesday
Monday is less effective because most people are still in a 5hitty mood at having to go back to work after the weekend, and Thursday seems to be the start of the weekend for most people these days.
Friday? People's minds are just elsewhere, especially after 11am, which is when most of the mail gets delivered here in the UK.
It goes without saying that there's no other changes to the mailing package.
I'm just looking for feedback on the most responsive delivery day, and I'm happy to be proved wrong on my assumptions...