Pondicherry is a great place to visit during the Christmas and for good reasons too. This is a former French colony, and to this day, there is a decent expat population living here. Some of them who have gone back make it a point to come back during Christmas, and celebrate their time here on the special occasion. So it becomes something like a mini France. With so many Christians living in the city, it would certainly be a different kind of experience celebrating Christmas among them.
Although according to the popular belief, Christmas in Goa is the only option for Christians due to many churches there but the truth is that you will find a lot of churches in Pondicherry as well like Goa and some other places with a good Christian population. And of course these churches are all decorated during Christmas. Do make it a point to visit at least a couple of these churches and attend the mass. This will definitely be a unique experience. Also, listen to those singers singing carols. You will find the French too singing these carols along with Indian Christians. There will be Christmas trees everywhere too.
The next thing to experience in the city is its fabulous dinners during Christmas. There are many restaurants here that offer the traditional turkey dinner during the festive season.