month of Sundays, a - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A long time, as in I haven't seen Barbara in a month of Sundays. This expression, which would literally mean thirty weeks, has been used hyperbolically since it was first recorded in 1832. One writer suggests it originally connoted a long dreary time, since games and other kinds of amusement used to be forbidden on Sunday.