This is a poster to advertise "Sonic & Tails 2" (the Game Gear game), but it is also a calendar. See the tiny numbers all along the right side? They're all the months/days for 1995, which was when the game was released in Japan. The calendar's function was likely to let the poster still be useful/functional after the game is released in hopes that it would stay on the wall anyway.
The poster has the title art for the game, Knuckles sort of randomly in one corner, giant Eggman head rising up at the bottom & various scattered little Sonics. (Look at the top, there's Knack) There are also 3 screen shots from the game & possibly a small explainer headline bit in the bottom right. Over all, the poster is colorful, fits in with the game box-look, & has the added interest of the calendar- with that much nice character art & design, it's a cool collectible now.