If it was a Monday on 1 January 2018, what was the day of the week on 31 December 2020?

Thursday

The correct answer is Option (4) → Thursday

1. Identify the Years and Their "Odd Days"

A normal year has 365 days. Since $365 \div 7 = 52$ weeks and 1 day, a normal year has 1 odd day.

A leap year has 366 days. Since $366 \div 7 = 52$ weeks and 2 days, a leap year has 2 odd days.

• 2018: Normal year (365 days) $\rightarrow$ 1 odd day
• 2019: Normal year (365 days) $\rightarrow$ 1 odd day
• 2020: Leap year (366 days) $\rightarrow$ 2 odd days (2020 is divisible by 4)

2. Calculate the Shift in Days

We want to find the day on 31 December 2020, starting from 1 January 2018.

• 31 Dec 2018: In a normal year, the first day (1 Jan) and the last day (31 Dec) are the same. Since 1 Jan 2018 was a Monday, 31 Dec 2018 was also a Monday.
• 31 Dec 2019: This is exactly 365 days (1 normal year) after 31 Dec 2018.

$\text{Monday} + 1 \text{ odd day} = \mathbf{Tuesday}$

• 31 Dec 2020: This is exactly 366 days (1 leap year) after 31 Dec 2019.

$\text{Tuesday} + 2 \text{ odd days} = \mathbf{Thursday}$

3. Conclusion

Starting from Monday (1 Jan 2018), and moving through the cumulative odd days of the subsequent years, we find that the day of the week on 31 December 2020 was Thursday.