1. In 1978, "August 1st" was a Tuesday, August 1978 had 31 days, and September 1978 had 30 days. What day of the week was National Day in 1978?
1. Solution: From "August 1st" to National Day there are 61 days, since $61 \equiv 5(\bmod 7)$, and "August 1st" is a Tuesday, therefore National Day is a Sunday.