If it was Monday on 19th July 2021, then what will be the day on 19th December 2021? 

Total number of days from19th July 2021 to 19th December 2021: So, number of days in the given months, From 19 th July 2021 to 18 th August 2021: 31 days From 19 th August 2021 to 18 th September 2021: 31 days From 19 th September 2021 to 18 th October 2021: 30 days From 19 th October 2021 to 18 th November 2021: 31 days From 19 th November 2021 to 18 th December 2021: 30 days Total number of days= (31+31+30+31+30) = 153 days The day of the week repeats itself in 7 days. So, the number of odd days = 153÷7= 6 (Number of odd days is equal to the remainder) So, it was Sunday on 19 th December 2021. (Monday + 6 odd days = Sunday).