What are the three days in Tomb-Sweeping Day?

Tomb-Sweeping Day is usually on April 4th, 5th or 6th of the lunar calendar.

Generally speaking, Tomb-Sweeping Day falls on April 4th, because the 24 solar terms may be pushed to April 5th or April 6th. Anyway, it's three days in Tomb-Sweeping Day. I remember that this year's Tomb-Sweeping Day was April 5th, but there was also a year when it was April 6th.