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

January 1, 2000 was a Saturday.

To find the day of the week for January 1, 2001, we need to determine how many days passed between these two dates.

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The year 2000 is a leap year because it is divisible by 400.

A leap year has 366 days.

Therefore, from January 1, 2000 to January 1, 2001, there are 366 days.

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To find the day of the week, we calculate how many complete weeks are in 366 days:

$366 = 52 \times 7 + 2$

$366 = 364 + 2$

This means 366 days = 52 complete weeks + 2 extra days.

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Starting from Saturday and adding 2 days:

Saturday + 1 day = Sunday

Saturday + 2 days = Monday

Therefore, January 1, 2001 was a Monday.

The answer is Option 1: Monday.