Praise be to Allaah.
If these celebrations are the festivals of the kuffaar and mushrikeen, it is not permissible to take part in those innovated festivals, because doing so implies helping them to commit sin and transgression. Taking part in their festivals also involves imitating the kuffaar, which is forbidden in Islam. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever imitates a people is one of them.” (Reported by Abu Dawood and Ahmad). ‘Umar (may Allaah be pleased with him) used to say: “Avoid the enemies of Allaah during their festivals.” (Reported by al-Bayhaqi).
If what is referred to is accepting an invitation to a meal, for example, and there is nothing involved that is forbidden in Islam, such as mixing of men and women, or foods forbidden by Allaah such as wine and pork, or dancing and music and so on, and this participation does not imply that one loves these kuffaar, then there is nothing wrong with accepting their invitation. He should try to convey the message of Islam to them. The Prophet (peace and blessings of Allaah be upon him) accepted the invitation of some of the Jews. And Allaah knows best.