"Nomination Period: April 1 through May 15 of an even numbered year. The prize will be awarded the January after nominations close, which falls in an odd year."

See for example the dates on the various announcement notices as given in the notes in the Wikipedia article: https://en.wikipedia.org/wiki/Joan_%26_Joseph_Birman_Researc...

(Note also that 2020 may have been unusual because pandemic.)