Is celebrating and offering congratulations for birthdays and marriages considered a custom or an act of worship?

The documentation for prohibiting the celebration of these holidays is not because they are acts of worship by which the celebrant draws closer to Allah Almighty. Rather, it is a prohibition of adopting holidays not stipulated in the Sharia, even if they are customs and not acts of worship. This is because the Prophet, peace and blessings be upon him, did not approve of the people of Madinah taking two days for play, even though they did not consider them as part of their religion, but rather as a custom they were accustomed to before Islam. This is indicated by the statement of Sheikh Ibn Uthaymeen: "Specifying days, months, or years for a feast is a matter referred to the Sharia, not to custom. Therefore, when the Prophet, peace and blessings be upon him, came to Madinah and they had two days on which they would play, he asked: 'What are these two days?' They said: 'We used to play on them during the Jahiliyyah.' Then the Messenger of Allah, peace and blessings be upon him, said: 'Indeed, Allah has replaced them for you with better days: the Day of Adha and the Day of Fitr.'"