In ("The Anniversary Present" - Season 2, Episode 3) the Ricardos' anniversary is on the 19th. But, in this episode it is on the 7th.
Based on the date of the Ricardo's anniversary given in this episode, it's sometime in early April, but Fred and Ricky say that if they were back in NY they would be begging for more heat and shoveling snow. Even in NY, which has a much colder climate than California, the chance of snow in New York City in April is pretty slim.
In the Mocambo, Ricky whispers something to the maitre d' and then the maitre d' whispers to the lead violinist, who invites Ricky to come up. Ricky then gives his speech about how it was his anniversary and he forgot it, but he hopes "this one does it." The band then begins playing the Anniversary Waltz as Ricky sings, but they were never told what to play.