Also known as: 2月10日はにとりの日, にとりの日, 2月11日はにとりの日に間に合わなかった人の日
February 10th is Kawashiro Nitori Day, one of Touhou Project date tags. Since February 10th can be read as "2 (ni) 10 (to)" it's a day when artists draw and post artworks with Nitori.
For those who missed February 10th, there is also a tag called "February 11th is the day for those who missed Nitori Day."