Noob day is where gamers have received a new video game as a Christmas gift and are playing it for the 1st time.

Got the covid-19 vaccine

ah yes. this refers to.. well just look at the name. back then when you and i were a wee-little-nooblet. it is a reflection of the "stone age" if you will.
used obviously in gamer language, this is more of a historical double word. just remember way back when you used "heavies" and noob weps and everyone criticized you for your lame fast kills. its all writen in the books. its all there.
the noob-days start once you enter a new game and dont know how to play.. as you go... they fade away more and more. sigh... good times... dark goooood times.