A calendar knows exactly when it stops being useful, and it prints the date on itself.
That makes it a strange commission: something built to be thrown out, that still has to earn a wall for the whole run first. Nobody keeps last year's, but everybody looked at this year's most days — same wall, same glance on the way past. So it has to survive being background: present without being noticed, useful without being read, and then binned in January without a second thought.
The real constraint is the grid. A calendar is a design in permanent negotiation with a table of numbers you didn't get to place, and that table comes back every page, unchanged. Your artwork shares the sheet with it or it doesn't run at all. The binding takes the top edge — the spiral punches straight through where a header wants to go — and on a wall calendar the page flips up and hangs, splitting image from grid at a crease you have to design around. The trap is consistency: the picture changes every month, the system can't wobble once.
Calendars live in the parts of a building nobody decorates on purpose: kitchen walls, garage doors, the side of a filing cabinet, the desk corner by the phone. Half of them are giveaways — the branded wall calendar a local firm hands out in December is a year of advertising nobody throws away too early. The shapes vary more than the grid does: wall calendars that fold over, tent-style desk ones on an easel, tear-off daily blocks, poster year planners, slim pocket ones. In Britain the appointment book is a diary and the grid on the wall is the calendar; Americans reach for planner instead.