Volume 3. Issue 4.
What was said? It will take you a month of Sundays to get through all the revisions.
Did someone really say that? Yes, in a client meeting discussing product promotions at retailers and the many revisions that are made before the promotion goes live.
What does it mean? Something that is going to take a LONG TIME…. A seemingly endless or prolonged period of time.
Origin: The expression is said to mean 30/31 weeks (the amount of time it takes a month of Sundays to pass) and has is believed to have origins from the Christian Holy Day of Sunday, the Sabbath. This day was a “day of rest” and was a long, solemn day devoid of amusement. Activities were even regulated on Sunday by law at times and therefore Sunday could seem long and tiresome (out of boredom)… therefore a month of Sundays could feel like an eternity. It is also sometimes used to denote something that will never happen.
The Oxford English Dictionary cites the first printed use of the phrase from 1759:
“The commander..swore he should dance to the second part of the same tune, for a month of Sundays.”
H. MURRAY Life & Real Adventures Hamilton Murray I. x. 121
NOTE: There are some variations on this, such as: Week of Sundays, Week of Saturdays, etc.
A whole month of Sundays
Sources:
http://dictionary.reference.com/browse/month+of+sundays,+a
http://www.phrases.org.uk/bulletin_board/59/messages/129.html
http://www.wordwizard.com/phpbb3/viewtopic.php?t=20154
http://www.wisegeek.com/what-does-the-idiom-a-month-of-sundays-mean.htm#did-you-know
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HOT Idioms 
They say an awful lot of idioms in your office! I love your blog!!
Hi, did you really mean to say 30-31 weeks, or did you mean 31-31 days, but each like a Sunday, as in the calendar illustration you included?
i just used this. two more times, and it’s mine!!
The misochristian overtones in your explanation make you seem a bit tone-deaf. It’d be a century of Sundays before I’d consider working in your office.