If tomorrow is monday, today is sunday.
A week has 7 days " Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday ".
64 / 7 = 9 ( Remainder = 1)
So you can say that after multiples of 7 days, the day will repeat itself as sunday.
Hence after 63 days sunday will occur again.
So we calculate after the remainder (1) , the answer is Monday.
Simple Logic = And whatever the remainder obtained is, add that to the day.