Can't Buddhists share a bed on the fifteenth day of the first month?

Yes, according to Buddhist precepts, the sexual life of disciples at home is defined as seven aspects: non-human, illegal, non-dependent, untimely, non-local, non-quantitative and irrational. The question you ask is untimely, that is, you don't have sex within the specified time, which includes the sexual behavior of husband and wife from sunrise to the day before sunset, the tenth day of each month and the lunar calendar every year.

It can be seen that the fifteenth day of the first month belongs to the tenth day of fasting, so you can't have sex, otherwise you will commit adultery.