If it was Saturday on January 1, 2000, then January 1, 2001, would have been:

Monday

The correct answer is Option (1) → Monday

To find the day of the week for January 1, 2001, we need to calculate the number of "odd days" between the two dates.

1. Identify the Year Type

First, we check if the year 2000 was a leap year:

• A century year is a leap year if it is divisible by 400.
• $2000 \div 400 = 5$ (No remainder).
• Therefore, 2000 was a leap year.

2. Calculate the Odd Days

In a calendar:

• An ordinary year has 365 days (52 weeks + 1 odd day).
• A leap year has 366 days (52 weeks + 2 odd days).

Since the period from January 1, 2000, to January 1, 2001, includes February 29, 2000, we must account for the extra day. This means there are 2 odd days to add to the starting day.

3. Determine the Day

• Start Day: Saturday
• Calculation: Saturday + 2 days
• Result: Monday (Saturday $\rightarrow$ Sunday $\rightarrow$ Monday)

Conclusion

January 1, 2001, was a Monday.