Calendars are like a 24 hour commercial, without the annoying jingle. Think about it, most people spend an average of nine hours a day in their office. During that time they'll look at their calendar three or four times.
They work five days a week, sometimes coming in on a weekend.
That's a lot exposure to your message.
Calendars are not expensive so you're getting multiple, multiple exposure for very little money.
Calendars are a very cost effective promotional advertising product