If tomorrow is thursday, today is wednesday.
A week has 7 days " Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday ".
18 / 7 = 2 ( Remainder = 4)
So you can say that after multiples of 7 days, the day will repeat itself as wednesday.
Hence after 14 days wednesday will occur again.
So we calculate after the remainder (4) , the answer is Sunday.
Simple Logic = And whatever the remainder obtained is, add that to the day.