Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
BIRTH'DAYS . The custom of making a feast in anniversary of a birth is very ancient. We find reference to it in Gen 40:20. In regard to the custom in Egypt, Wilkinson tells us: "The birthdays of the kings were celebrated with great pomp. They were looked upon as holy, no business was done upon them, and all classes indulged in the festivities suitable to the occasion. " "The day of our king," spoken of in Hos 7:5, was probably his birthday. It was upon Herod's birthday that John Baptist was beheaded. Matt 14:6-10.
The fact that the Herodian family observed birthdays would be an additional grievance on the part of the Jews.