Noun: All Hallow’s Day, All Saints’ Day.
Traditionally, this holiday occurs on November 1st. In Finland it is celebrated on the first Saturday of November.
Pyhä is an adjective that means ”holy” or ”sacred,” and when capitalized it means ”saint.” Pyhäin is a rare form of the genitive plural of pyhä and means ”Saints'”
Read more about Pyhäinpäivä here.