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

Saturday

The correct answer is Option (1) → Saturday

Step 1: Calculate the Number of Days/Years

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

• 2007: Ordinary year (365 days) = 52 weeks + 1 odd day
• 2008: Leap year (366 days) = 52 weeks + 2 odd days
• 2009: Ordinary year (365 days) = 52 weeks + 1 odd day
• 2010: Ordinary year (365 days) = 52 weeks + 1 odd day

(Note: 2008 is a leap year because it is divisible by 4, meaning it includes February 29th.)

Step 2: Total Odd Days

Sum of odd days = $1 + 2 + 1 + 1 = \mathbf{5}$ odd days.

Step 3: Determine the New Day

Since we are moving forward in time, we add these 5 odd days to the starting day (Monday):

1. Monday + 1 = Tuesday
2. Monday + 2 = Wednesday
3. Monday + 3 = Thursday
4. Monday + 4 = Friday
5. Monday + 5 = Saturday

The day of the week on January 1, 2011, was Saturday.