Explanation:

1600 years give no odd days
200 years give 3 odd days
36 years give (36 + 9 ) or 3 odd days
Thus 1836 years give 6 odd days
From 1st January to 20th June there are 3 odd days
Odd days: January 3, Feb. 0, March 3, April 2, May 3, June 6 = 17
Thus the total number of odd days = 6 + 3 or 2 odd days
This means that the 21th of June fell on 3rd day commencing from Monday.
The required day was Wednesday.