Calendars are one of the oldest forms of promotional products. (The earliest known use of a promotional product was when Coshocton, Ohio newspaperman Jasper Meeks printed a book bag with the name of a local shoe store).
Calendars have one of the best returns on investment, hanging or sitting in front of clients for 12 months for as minimal investment. People tend to look at calendars several times a day, so the cost per exposure is very low, compared to some other forms of advertising.
I remember the calendar that hung in my Grandparents' home. It was from a local funeral home, and hung beside the phone. For years, that imprinted calendar would be replaced with a new version, same picture and format. And for years, my family has used that funeral home whenever we had the need.
Advertising doesn't have to be expensive to be effective. Calendars are the time tested proof of that.