Hey, welcome to the community!
I’m not part of the OpenAI team, just another member like you.
I think Monday got popular because it was made by the ChatGPT team and pinned to the sidebar. Since users couldn’t remove it for about a week, a lot of people ended up trying it out, even just out of curiosity.
It could not be unpinned right away, so it stayed visible for a while due to a bug (maybe intentionally made by OpenAI 
).
But really, Monday is just one custom GPT.
There are thousands of others out there that give similar replies, some even more sarcastic.