It was Monday on January 1, 2007. What was the day of the week on January 1, 2011?

Given that January 1, 2007 was a Monday, we need to find the day of the week on January 1, 2011.

The time period spans 4 complete years: 2007, 2008, 2009, and 2010.

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To determine the total number of days, we need to check which years are leap years.

A year is a leap year if it is divisible by 4.

2007: Not divisible by 4 → 365 days

2008: Divisible by 4 → 366 days (leap year)

2009: Not divisible by 4 → 365 days

2010: Not divisible by 4 → 365 days

Total days $= 365 + 366 + 365 + 365 = 1461$ days

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To find the day of the week, we calculate $1461 \mod 7$:

$1461 = 208 \times 7 + 5$

This means 1461 days $= 208$ weeks $+ 5$ days

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Starting from Monday (January 1, 2007) and adding 5 days:

Monday $\to$ Tuesday $\to$ Wednesday $\to$ Thursday $\to$ Friday $\to$ Saturday

Therefore, January 1, 2011 was a Saturday.

The answer is Option 1: Saturday.