Shavuot is the holiday celebrating the anniversary of receiving the Torah and G-d Almighty instructing us the 10 Commandments.
On this day, it is a mitzvah for men, women, and children to hear the 10 commandments read from a Torah scroll, similar to the way we heard them 3,335 years ago.
Also, it’s an old custom to eat dairy foods on Shavuot.
Amongst the reasons for this tradition are:
1) On this day, when G-d gave us the Torah, we were elevated to a higher level of holiness. For this new level of holiness, the animals ritually slaughtered earlier were no longer fit for us. And the Torah was given to us on Shabat when it is forbidden to slaughter new animals ritually. Hence, not being able to eat meat, we ate dairy.
2) Every letter in the Hebrew alphabet also has a numerical value. The Hebrew word for dairy is “chalav.” Its numerical value is 40. This corresponds to the 40 days and 40 nights Moses spent on Mount Sinai studying the Torah with G-d, following G-d giving us the 10 Commandments.