Hello Dhanya,
Wow a trip to Swiss is going to be unforgettable.Be ready to beat the snowy winter with loads of warm clothes and boots are a must.
Most of the shops will be closed only on the 25th.So enjoy your shopping there amidst watches, chocs and lovely woolies.We stayed in Engelberg , very close to Mt.Titlis during Christmas and had fabulous time there.Interlaken I am not sure if it will be open on Chirstmas day but as I said all the shops and tourist places are open on other days during the winter.They are not closed for the whole month of december.
Wish you a safe and happy journey and don't forget to post some pics when you are back ..