The Emperor's Birthday is a holiday celebrated on three planets, but not in this room. In here, Gregor doesn't celebrate anything. He's a year older, but that hasn't been important since his twentieth, and that was terrifying. His birthday has always been a celebration for other people. His birth was a gift to them and so is his continued existence. Gregor doesn't think that's worth celebrating.
The personal gifts he receives are mostly given out of duty, and he cherishes the ones that aren't. He puts them here in this room where the Emperor's Birthday cannot reach. Only he can.