for some it has to be early Monday morning others say people tend to be busy early morning as they have enough work load.
Fridays is avoided as again people are in weekend mood so they do not pay any attention to it.
Smart ones think of sending it on Wednesday afternoon thinking its a balance of median wee time.
Numerous Theories are flooded in the Virtual world each trying to prove his point and giving Justification for the same.
Some of the key things that need consideration are
Categorizing Your Target market into various class of people, Your experience about them, Country, Class of people, Gender, Age group whether Business, Professionals, Students etc.
what are the best days and times to Tweet, update blog, send email(s), update Facebook pages or personal postings etc.