Hi Ritu. Welcome to DC.
Freshly-made is always best, but try wrapping the sandwich in saran wrap & keeping it in the fridge - one or two days tops. Some markets have sammiches in those plastic divider containers that are sealed with an expiration date. Mayo & relish might make the bread soggy. If you're pressed for time, make up the sammich fillings (i.e. tuna/egg/ham salad etc.) ahead & put it in a sealed container or a food sealer bag. A zip lock might help.