It was a Sunday on 1 January 2006. Using this information and standard calendar rules, what was the day of the week on 1 January 2010?

Introduction:
This is a multi-year calendar question. We are given the weekday for 1 January 2006 and asked to find the weekday for 1 January 2010. Such questions test your understanding of how leap years and non-leap years shift weekdays over several years.

Given Data / Assumptions:
1 January 2006 is Sunday. We need the weekday on 1 January 2010. We consider years 2006, 2007, 2008, 2009 leading up to 2010. We use standard leap-year rules.

Concept / Approach:
Each non-leap year has 365 days, which is 1 more than a full number of weeks (364 = 7 * 52), so it shifts the weekday of the next year by +1 day. Each leap year has 366 days, which is 2 more than a full number of weeks, so it shifts by +2 days. By counting how many non-leap and leap years occur between 2006 and 2010, we can determine the total shift in weekdays.

Step-by-Step Solution:
Step 1: List the years from 2006 to 2009. 2006, 2007, 2008, 2009 are the years completed before reaching 1 January 2010. Step 2: Identify leap years. A year is leap if divisible by 4 (and not a century year that is not divisible by 400). Among these, 2008 is a leap year. 2006, 2007, 2009 are non-leap years. Step 3: Compute total weekday shift. Each non-leap year shifts by +1 day: 3 non-leap years ⇒ +3 days. The single leap year (2008) shifts by +2 days ⇒ +2 days. Total shift = 3 + 2 = 5 days. Step 4: Apply the shift to the starting weekday. Start: 1 January 2006 = Sunday. Move forward 5 days: Sunday → Monday (1), Tuesday (2), Wednesday (3), Thursday (4), Friday (5). So, 1 January 2010 is a Friday.

Verification / Alternative check:
You can verify by building a small table year by year: 1 Jan 2007 (Monday), 1 Jan 2008 (Tuesday), 1 Jan 2009 (Thursday) due to the leap year in 2008, and finally 1 Jan 2010 (Friday). This matches our total shift approach.

Why Other Options Are Wrong:
Sunday, Saturday, Wednesday, Monday: Each of these would correspond to a wrong total shift of 0, 6, 3, or 1 days from Sunday, respectively, which does not match the actual shift of 5 days produced by the sequence of leap and non-leap years.

Common Pitfalls:
Common mistakes include forgetting to treat leap years as +2-day shifts or miscounting which years in the interval are leap years. Some candidates also forget that we count complete years between the two dates, not including the final year as a full year if we are going to its first day.

Final Answer:
1 January 2010 fell on a Friday.