Question: What is the reason for the custom of eating dairy foods on Shavuos?
Answer: There are numerous explanations given for this interesting custom. Here are just a few. These undoubtedly contain allusions to deeper concepts beyond the scope of this response, but there is still much to appreciate on the surface.
(1) The verse that speaks of the bringing of the Bikkurim (Offering of the First Fruits, which are brought on Shavuos) also forbids mixing meat and milk, suggesting that both were eaten then (but not together).
(2) Mt. Sinai is called referred to as “Har Gavnunim” (Tehilim 68:16), and the word for cheese in Hebrew is “Gevinah”, a word closely related to “Gavnunim.”
(3) The initial letters of describing the offering brought on Shavuos are “[Minchah] Chadashah La-Shem Be-Shavuoseichem” spell “ChaLaV”, the Hebrew word for milk.
(4) The numerical value of Chalav is 40, alluding to the days Moses spent on the mountain.
Wishing you a Happy Shavuos!
Rabbi Azriel Schreiber