Here are some threads on the same topic from the "Popular Questions (FAQ)" section:

https://www.reddit.com/r/AskHistorians/comments/13zo9q/were_there_any_new_millenium_celebrations_in_the/

https://www.reddit.com/r/AskHistorians/comments/1powhu/did_people_in_999_ad_celebrate_the_new_millennium/

(somewhat unrelated) https://www.reddit.com/r/AskHistorians/comments/20uutu/the_year_1000_critics_modern_and_medieval/

Apologies if this breaks the rules by not contributing, delete if necessary please.