is it always Monday?
I know it is a trivial error but i think there is a slightly bigger one with DD - its still trivial.
Jo has said DD was born in 1881 so he would have left school in 1899 just before his 18th birthday. it was this summer when he met Grindelwald. After 2 months Gellert goes back to mainland Europe where he puts his ideas into practice for the next 5 years, because it was 5 years later when DD actually faced Grindelwald to defeat him which would make it 1904 but DD is supposed to defeat Grindelwald in 1945. Unless Gellert goes underground for 40 years where no one really knows where he is.
I know it is trivial in a way and it doesn't really matter but i need to know the dates in my head and have it all sorted out in my head.