While calendars are usually 'housewares', this one is specifically a "School Calendar", so here it goes on Sonic School Supplies. This is for the year 1995, and was sold only at a Japan Sega World.
But what's up with it? It doesn't look at all like a normal wall, desk, or mini calendar. It looks like it is a thick plastic yellow case, there's no sign of months or dates, and why does it have buttons? The black thing is a stylus. There's a dark blue instruction sheet, but you can see that it's all in Japanese. Is the dates/part under that spotlight themed top bit and you lift it month to month like regular ones? Of course, if you know what's going on with this item, you can write in for credit.

The calendar is on a Sega Pico, which was a kidde item made by Sega that didn't do all that well in the USA, but did get 2 sort-of-Sonic games. (Tails Music Maker & Sonic Gameworld--both of which you can emuluate) You can read more about the Sega Pico at Sega Retro link. But, it was never known for having a calendar.
Also notice that if you see a pico on that Sega Retro page, it's not the same as the one here as apparently they were usually hinged in the middle, the stylus was on a cord & that white area was larger. In this instance the calendar would be like a 'cartridge' because the Pico could interact with it when it was placed on that top part there. This Blog of Gameover Johnny has more Pico info. Pico information provided by: Pavel

TODO = backup links