Eating one on your birthday means that this year is as flawless as an egg, and the most important thing is that this year has passed like an egg, which is said to be rolling luck.
The so-called son's birthday is a bitter day for the mother, and there must be two eggs:
One for mom and one for yourself! Be sure to wait until mom finishes eating before eating by herself! In short, this is just a traditional custom.
Extended data:
There is also a folk saying that it is an allusion in The Book of Songs, which tells that in ancient times, a woman named Zhu Die met a swallow egg when she went out to play, and after swallowing it, she became pregnant and gave birth to a "deed".
Later, "Qi" became the ancestor of Yin Shang. Qi is an excellent person. Therefore, people in later generations hope that their children are as excellent as him, so they send swallow eggs to those who give birth to children to express their blessings. But swallow eggs are hard to find, so eggs are used instead.
References:
Baidu Encyclopedia-Eating Eggs on Birthday