Explanation: The week day advances by 1 for a non-leap year and by 2 for a leap year. Starting from 1st Jan 2000, every group of 4 years has exactly 1 leap year, so the week day will advance by 5 days.
From 2000 upto 2099, we have 25 such groups of 4 years; so the week day will advance by 5x25=125 days. From this, we can safely remove all multiple of 7; leaving only the remainder as 6.
Hence, in these 100 years the week day will go back by 1; from Saturday to Friday