According to an article in Target Marketing (12/11, p. 7), 56% of e-mail is sent Tuesday, Wednesday, and Thursday, distributed almost evenly among those 3 days.

Participants at a 12/8/11 Specialized Information Publishers Association (SIPA) Roundtable reported some success e-mailing on other days, most notably Friday and Saturday.

I send 4 e-mails to my list weekly. Monday and Thursday are content messages. Tuesday and Friday are promotions.

As a rule of thumb, at least 50% of your e-mail messages should be content. If not, people unsubscribe.