A calendar earns a place above a modelling bench for a second reason beyond the date: it keeps good technical photography in front of you all year. The listings cover large-format wall calendars and desk versions, mostly built around aviation photography. In practical terms they differ chiefly in image size and in how much wall or desk space they take, and a large sheet shows surface texture and camouflage edges that vanish in a small book illustration. Earlier editions keep their value as reference once the year has passed. A few notes from experience follow.
Good technical photography printed large shows what disappears in a small book illustration: surface structure, the soft boundary of a sprayed camouflage edge, exhaust staining, the way light breaks across a canopy. It is no substitute for a monograph, but as a running reminder of how the real thing looks it works well, especially when you are painting under artificial light and lose perspective.
A large wall calendar gives the photograph room and can be hung where you will actually see it while working. A desk version takes less space and suits a bench already occupied by a build and shelves above it. The difference is purely practical; the two do not exclude each other.
The emphasis here is on aircraft, which matches what modellers build most. If you want a particular theme, read the description of the individual calendar - each states its subject and the year it covers.
A calendar is tied to a year; the photographs are not. Earlier editions remain perfectly good visual reference once the year has passed, and some builders cut out individual sheets and file them with the kit for that type.
If your bench stands against a wall, hang the calendar at eye level when seated. Under a ceiling light it will otherwise catch reflections and the photograph turns into a glossy blur. Good task lighting on the model and indirect light on the reference is the arrangement that works.
For reference you can actually build from rather than look at, continue to reference books on real machines or back to the full modelling literature section.