It is permissible to treat Christian people who are at close proximity to you, neighbors or classmates, gently and help them do lawful things. It is also permissible to visit them with the intention of calling them to the way of Allah, so that Allah may guide them to Islam. However, joining their festivals or congratulating them on such occasions is impermissible. Allah, (Glorified be He) says,
Help you one another in Al-Birr and At-Taqwâ (virtue, righteousness and piety); but do not help one another in sin and transgression.
Joining their festivals, as well as taking them as intimate friends, is regarded a form of forbidden alliance.May Allah grant us success. May Peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!