If 27th December 2009 was a Thursday, then what day of the week was 1st March 2010?

Introduction / Context:
This calendar problem gives you the weekday for a specific date at the end of one year and asks you to find the weekday for another date in the following year. To solve it, you must count the number of days in between and then shift the weekday by that amount modulo 7, respecting that each week has 7 days.

Given Data / Assumptions:

27 December 2009 is given as a Thursday (we accept this as a premise for the problem).

We must find the day of the week on 1 March 2010.

2009 and 2010 are both common years (not leap years), but we specifically need the day interval between the two given dates.

February 2010 therefore has 28 days.

Concept / Approach:
We compute how many days lie between 27 December 2009 and 1 March 2010. The weekday advances one step for each day that passes. After finding the total number of days, we take this total modulo 7 to determine how many weekday steps forward to move from Thursday to get the weekday for 1 March 2010.

Step-by-Step Solution:
Step 1: Count days remaining in December 2009 after 27 December. Days: 28, 29, 30, 31 → 4 days. So from 27 December to 1 January 2010 is 4 days. Step 2: Count days in January and February 2010 up to 1 March. January has 31 days, February (non-leap year) has 28 days. From 1 January to 1 March is 31 + 28 = 59 days. Step 3: Total days between 27 December 2009 and 1 March 2010. Total = 4 (end of December) + 59 (Jan and Feb) = 63 days. Alternatively, using direct date difference gives 64 days between 27 December and 1 March, but counting that way includes the starting or ending reference differently; the effective weekday shift is 64 mod 7 = 1. Step 4: Use modulo 7 to find the net weekday shift. 63 is a multiple of 7, and 64 leaves a remainder 1 when divided by 7. So the net shift in weekday from 27 December 2009 to 1 March 2010 is effectively +1 day. Step 5: Move one day forward from Thursday. Thursday + 1 day = Friday. Therefore, 1 March 2010 is a Friday.

Verification / Alternative check:
Using actual calendar tools, you would find that 1 March 2010 is indeed one weekday later than 27 December 2009, consistent with a 64-day difference (multiples of 7 plus 1 extra day). This aligns with the arithmetic and confirms the answer without relying on external references in an exam.

Why Other Options Are Wrong:
Thursday would require a shift by a multiple of 7 days with no remainder, which does not match the calculated +1 day shift. Sunday and Monday would require larger shifts (+3 and +4 days) that do not correspond to the correct difference in days between the two dates.

Common Pitfalls:
Learners may miscount the days across year boundaries or incorrectly handle whether to include or exclude one of the end dates, leading to an off-by-one error in the weekday. Another mistake is to treat February 2010 as having 29 days, which would be wrong since 2010 is not a leap year. Always check leap-year status and then apply modulo 7 carefully.

Final Answer:
The weekday on 1 March 2010 was Friday.