Celebrations and Customs

Q: I wanted to know that if a relative invites me for a dinner party which is for celebrating his sons birthday then should I avoid the invitation or should I join the dinner party and avoid the birthday celebration?

Q: What is the ruling regarding birthdays? Our muslims greet one another on this day and feel insulted if one does not greet them. Another issue is regarding birthday cakes, gifts and parties. Please advise.

Q: What is the ruling with regards to Easter celebrations, Easter eggs, hot cross buns, etc.?

Q: Is it permissible to tell someone "happy birthday"? I know its not a day to celebrate, nor do we do so but people feel appreciated / acknowledged that at least he remembered my birthday. Sometimes, some people take offense if you "forget" to tell them?