The expression "a month of Sundays" means an indeterminately great length of time before something happens, if it happens at all.

A month is about 30 days; therefore, 30 Sundays each seven days from the previous would be 210 days or seven-and-a-half months. But it is unlikely anyone using the expression has any such precise period in mind.

The saying was first used in 1832 by the novelist Frederick Marryat who wrote in Newton Forster: "In a minute Thompson made his appearance on deck, and steadying himself by the weather topmast backstay, fixed his leaden eyes upon the land on the quarter...and said, ‘it’s a devil of a gale, sure enough. — It may last a month of Sundays.’"

Shops and places of entertainment are open on Sundays today. In a bygone era, Sunday, the Sabbath for many Christians, was a long, solemn day devoid of amusement – an obligation sometimes enforced by law! Fun and frivolity were strictly taboo. In fact, most Sunday activities, except attending church were frowned on or forbidden. Time may have hung heavily on people’s hands. So a month of Sundays might have seemed like a dark period of time which felt longer than it actually was because of boredom.

In the U.S., some ‘blue laws’ regulating activities on Sunday are still in effect in many states and, thus, for the less religiously inclined, the tedium of Sunday could feel like a long, slow, dreary time.