Has anyone ever said to you, "I haven't seen you in a month of Sundays"? Perhaps you have said it to someone without knowing how many days are in a month of Sundays.
Literally, a month of Sundays is a unit of time corresponding approximately to one cycle of the moon's phases, or about 30 or 31 days or 4 weeks.
Some months have 30 days. Some months have 31 days. February has 28 days. Even so, all months have at least four Sundays. Technically, to say a month of Sundays is to say 30 Sundays which would be 30 weeks or roughly 7 and 1/2 months or 210 days.
When people tell you they have not seen you in a month of Sundays, it simply means they have not seen you in a long time. No one will actually count 210 days on the calendar.
Figuratively, a month of Sundays is an indefinitely long period of time.
Hopefully, you won't let a month of Sundays past before seeing family and friends.