On 12 January 1980, it was Saturday. What day of the week was 12 January 1979?

Concept

For the same calendar date in consecutive years, the weekday shifts by the number of days in the intervening year modulo 7.

A common year has 365 days, and 365 = 52 × 7 + 1, so the same date in the next year moves forward by one weekday.

Application

1. The interval from 12 January 1979 to 12 January 1980 contains 365 days because 1979 was not a leap year.

2. Thus, 12 January 1980 is one weekday later than 12 January 1979.

3. Moving one weekday backward from Saturday gives Friday.

Cross-check

Since 365 mod 7 = 1, moving forward from Friday by one weekday reaches Saturday, matching the given date. Therefore, 12 January 1979 was Friday.