What was the day of the week on 31 December 2011?

Introduction / Context:
Determining the weekday for the last day of a particular year, such as 31 December 2011, is a standard calendar aptitude problem. It is useful because many other dates in the year can then be related to this final reference point. The question checks your understanding of odd days across months and your ability to work from a known reference date.

Given Data / Assumptions:

• Target date: 31 December 2011.
• We assume the Gregorian calendar system.
• We can use the known fact that 1 January 2011 was a Saturday.
• 2011 is a normal year with 365 days.

Concept / Approach:
Once you know the weekday of 1 January of a particular year, you can find the weekday of any date by counting the days in the months in between and computing odd days. Alternatively, you can observe that in a normal year, 31 December falls on the same weekday as 1 January, because 365 days correspond to one odd day and the extra day effectively brings you back to the same weekday for the first and last days.

Step-by-Step Solution:
Step 1: Recognise that 2011 is not a leap year, so it has 365 days. Step 2: In any normal year, 1 January and 31 December fall on the same weekday. Step 3: We use the known reference that 1 January 2011 was a Saturday. Step 4: Therefore, 31 December 2011 also falls on a Saturday. Step 5: Alternatively, you can count month by month, but the normal year property already gives the answer quickly.

Verification / Alternative check:
If you want a direct month wise check, you can sum days from 1 January to 31 December, which is 365. Dividing 365 by 7 gives 52 full weeks and 1 odd day. Hence the weekday cycles through a full number of weeks with an extra day, bringing you back to the same weekday for start and end of the year. Using this principle with 1 January 2011 as Saturday confirms that 31 December 2011 is also Saturday.

Why Other Options Are Wrong:
Monday: This is two days ahead of Saturday and would require a different odd day pattern.

Sunday: This is one day ahead of Saturday, not supported by the normal year structure here.

Friday: This is one day behind Saturday and would correspond to a different starting weekday than the one assumed.

Common Pitfalls:
Some students confuse leap and normal years and assume that the first and last days always match. In a leap year with 366 days, 1 January and 31 December are on consecutive weekdays, not the same one. Make sure you confirm whether the year is leap or normal before applying this shortcut.

Final Answer:
Hence, the day of the week on 31 December 2011 was Saturday.