How many odd days are there in 400 consecutive years of the Gregorian calendar?

Explanation

1. 01

   Each block of 100 years has 5 odd days, but the 400th year is a leap year, adding 1 odd day.

2. 02

   Total $= 4 \times 5 + 1 = 21$, and 21 is a multiple of 7, so 0 odd days.