According to the New York Times Archives on Sept 30, 1921 a benefit fundraiser was held at the Polo Grounds to honor Mathewson. A five inning game between Giant Old-Timers and the 1921 squad was held to be followed by a regular season game against the Braves which was rained out after the first inning. Searching "Matty Day" yielded an article reporting Matty's letter to Baseball Association of Writers Pres. Frederick Lieb thanking him for the success of "Matty Day".
On May 14, 1925 the Giants game versus Cincinnati was preceded by an Old Timers Celebration which featured the likes of Amos Rusie and Roger Connor and Dan Brouthers. Although the article referred to the National League's Golden Jubilee nary a mention was made of Mathewson.
Searching "Mathewson Day" the only hit I got was for an event held in Factoryville, Penna. held on August 29, 1923 which Matty was able to attend.
There is this in 1926:
BASEBALL TEAMS JOIN IN MATTY DAY GAMES
Every State in Nation Stages Contests to Aid in Building of Memorials.
August 29, 1926, Sunday
Baseball teams throughout the country observed Matty Day yesterday. Professional, semi-professional, industrial and amateur teams contributed the proceeds of their games to the Christy Mathewson Memorial Foundation to assist in the building of two memorials to the famous pitcher -- one at Bucknell College, Matty's alma mater, and the other at Saranac Lake, where he spent his last days.
Nothing definitive here. The NL began in 1876 if I am not mistaken hence the Golden Jubilee year would be 1926 yet the 1925 game referenced the GJ. The dates on the ball are a bit confusing and I am not sure what's up with "Christy Matty Day". The articles make for fun reading. They are in .pdf format and apparently too large to upload with the "Manage Attachments" application. E-mail me for copies.