I recall someone with a list of circa 20k or so telling me they had split test and found Friday's the best day.

I do suspect however it may depend on the nature of your audience and the nature of the offer emailed about.

Argument / reasoning for Fridays may be.. people are generally happier and perhaps more responsive.

Mondays...they are on more of a downer as it is typically first day back with the nose to the grindstone at work.

Then again if you happen to be in say the biz op business offering an escape from the 9-5 Monday may be the day when it makes most sense to the reader

I don't think there is one easy answer. You need to find the right answer for your particuar niche and target audience.

I have to admit to having little to know experience of this myself.
If you have a list big enough to warrent fine tuning sending however I would consider split testing. Here is a possible plan.

1 - take your mailing list then jumble it up then split it into 7 smaller lists. Do this jumbling else your results may be skewed due to list sign up freshness. New joiners may be more responsive than old etc.

2 - pick a week in mid month to try and minimise any monthly pay day effect.

3 - send exactly the same email at the same time of day to each of the 7 groups and monitor open rates etc.
Obviously you may need to set up 7 click tracking urls etc.

The results when analysed should give you an indication as to whats a good bet for your future mailings.

Feasibly month 2 you could retest but this time varying time of day as opposed to day of week.

Sounds like good advice. Just wish I bothered to do it all myself

Cheers
Mick