I think you misunderstand what celebrate means. Celebrate doesn’t imply joy by its nature, but rather commemoration, a remembering. You celebrate birthdays, baptisms, but also funerals, and war. When people remember the September 11th attacks and hold memorial services, or perform some kind of routine in remembrance of the day, they are celebrating.
The English name of “Easter” may have its origins in pagan lore, but it is how we celebrate it that matters. Not the name. When Christians refer to Easter, we refer to the day Jesus rose from the dead. And while many may also partake in the custom of the Easter bunny and the like, it is out of fun and not pagan observances. Many even take the pagan symbols and use them for Christian symbolism, replacing the pagan aspect to something Christ centered.