Idle musing … a “month of Sundays” is about 7 months (assume month has about 30 days, so you need 30 weeks to get 30 Sundays).
And “once in a blue moon” is a little rarer than that. A blue moon is the name given to the second full moon occuring in a calendar month – see NASA’s description for details. They occur, on average, once every two-and-a-half years. The next one will be in July 2004.