A month of Sundays

That's not a common expression; judging from Google, it seems to have been used only once, as the headline of a bbc.com slideshow about Paris in August: http://www.bbc.com/travel/slideshow/20130724-a-month-of-sundays-in-paris

The implication is that, in Paris, the whole month of August is like one extended Sunday: leisurely, no one is at work, no one is in a hurry.

The headline is meant to be clever, using the phrase "a month of Sundays" without its normal idiomatic meaning.

A month of Sundays doesn't mean something will never happen. It is generally just an expression used hyperbolically to indicate a long period of time, since a month of Sundays would be about 30 weeks. See here, here and here.

Occasionally, it's also used to indicate a period of inactivity or leisure as also key to it's meaning, given the nature of Sundays in the western tradition as a day of rest.