Some people say that when the Prophet (peace and blessings of Allah be upon him) was placed into his mother's womb (some other say, the day he was born) Allah declared a holiday for the angels!

This is a false and fabricated statement. It is not found in any reliable source.

Angels are a special creation of Allah. They are always loyal to Allah Ta'ala. They never disobey him. The angels are always engaged in the tasks Allah has assigned to them. And the angels are created by Noor. They do not need to eat or drink or rest. Thus, they have no need for holidays.

May Allah grant us Tawfiq to refrain from such fabricated statements - Ameen.