Festivities, festivals in Tokyo December

Are there any things specific things in December in Tokyo that you would recommend a tourist to go see or do?

Which Christmas market has the best atmosphere? Which ones are romantic or good for a date?

Thank you.

It's not in Tokyo, but just 1.5 hours train ride, Chichibu Yomatsuri (night festival) is very famous festival held in December 2nd and 3rd (The biggest event will be held on 3rd.)
It's on UNESCO’s Intangible Cultural Heritage list
https://www.japan-guide.com/e/e6552.html
https://matcha-jp.com/en/4197

There is no custom of doing Christmas market in Japan,
I found some in Tokyo and around, but I do not know if it will be done this year with the same name.
東京スカイツリー タウンドリームクリスマス - （東京都・東京スカイツリー）
六本木ヒルズアーテリジェントクリスマス - （東京都・六本木ヒルズ）
東京クリスマスマーケット - （東京都・日比谷公園）
クリスマスマーケット in 横浜赤レンガ倉庫 - （横浜市・横浜みなとみらい21）
https://www.ozmall.co.jp/xmas/xmasmarket/
https://www.enjoytokyo.jp/feature/christmas/event/article01.html
Last one in Yokohama city and Saitama city one are not in Tokyo as location address, but not so far.

It is too early to look for Christmas events now, still flood of info of Halloween.

BTW, are you a real Melonpanian, she also will be soon.
https://www.japan-guide.com/forum/quereadisplay.html?0+165030
The meaning of Melonpanian is the alien who came from the star of Melonpan :)
https://store.line.me/stickershop/product/1298671/ja
https://store.line.me/stickershop/product/1298671/en