Lucy points to a calendar showing where she circled her anniversary, the 19th. No month is visible on the calendar (the name appears to be covered with a postcard). The only month beginning on a Thursday in 1952 (the year this episode first aired) is May. Additionally, Friday, May 30th is highlighted on the calendar which would have been Memorial Day in 1952.
Ricky is reading the May 4, 1952 issue of "This Week Magazine". This episode was filmed five days later.