Great Northern Railway, UK, poster, 1904

Whitsun is another name for Pentecost, the seventh Sunday after Easter, and Whitsuntide is the following week. Obviously, the actual dates involved vary with the date of Easter.

The Scotch Express hauled by a C1 class 4-4-2 from the 1900s, a pic taken that decade. Of the 94 built, only one of this class, number 251, survives.