2021
Either the image has '2021' written somewhere on it, or it makes a reference to the year 2021.
Also when 2021 appears as a tag when searching through pixiv.
2021 was the Year of the Ox.
Do not tag if they are only part of a date!
See also
- (Postponed) 2020 summer olympics
- Comiket 99
- (Postponed) Euro 2020