Today is Tuesday April 3rd but it looks like it’s showing Monday April 3rd.
It is because the week on the calendar starts by… Monday.
So Sunday 01, Monday 02, Tuesday 03 etc…
I never understood calendars starting by Sunday 
. I think North America is the only place where I saw this, but maybe it exists in other countries.
@clementcg is correct. The heat map is using the international standard ISO 8601. Monday is the first day of the week and Sunday is the final, 7th day.
I’ll make a note of this thread and any comments we get for further consideration in any future localization efforts we undertake!