We always mail on Fridays, via First Class Mail, so that "most" if not all the mailers hit mailboxes the following Monday.
When the mailers hit on Monday, the recipients respond to the mailer by phoning in, usually beginning mid-day on Monday, then continuing Tues and Wednesday, then tapering off by the end of the week.
The upside to this is that if someone calls you back and leaves a message for you on Mon-Wed, you have until Friday to call them back before the weekend. You don't want to let too much time lapse between when they call and when you call them back, so it's best not to have a weekend timeframe stuck in there.
Plus, you can take care of all the week's leads in the same week, then schedule site visits for that following weekend (if you want).
The other upside to this is that it helps you measure your response rate more accurately. You send a mailing each week, it usually lands in mailboxes on Monday, then you can count how many responses you get during that week and attribute them all to the mailing that hit on Monday.
It's not an exact science, of course. You will always have those respondents who call you that week, but they received a mailer from you several weeks ago. I've had people call back as much as 6 months later.
Anyway, to answer your question - mail on Fridays, so it hits mailboxes on Monday.
(And don't mail more often than every 5-6 weeks to the same person.)