Aug 25, 2015i was told that both "21st century" and "21th century" were common expressions, but i don't think the latter is grammatically correct. In the twenty-first century || in the 21st century? What a heated discussion @sound shift you would say "my birthday is the 21st of december" , but does adding on feel wrong to you?
Jun 25, 2007early = 1st to 10th mid = 11th to 20th late = 21st to 30th beginning of (included in early) end of (included in late) = last 5 days = 26th to 30 th i also had the same impression, namely that. "we have a school trip on the afternoon of october 21st." thoughts.