I do not celebrate Christmas, Halloween, Easter, Lent, Passover, Feast of Tabernacles, Mid-Autumn Festival, Chinese New Year, etc. I don't even celebrate my birthday. I didn't attend my Bachelor's, Master's, or PhD convocations. Basically, I do not celebrate much but I am happy that others do :)
Romans 14:
Colossians 3:
I think it is okay for Christians to celebrate Halloween, Yom Kippur (Day of Atonement), and Rosh Hashanah (Feast of Trumpets). It is up to each one of us to decide guided by our conscience before God. I would not be a party pooper :)