As 95 percent of the country population is Muslim and 5 percent is mixture of Orthodox Christians and Jewish. So Muslims don't celebrate the Xmas as it is known but the Orthodox celebrate it in a slightly different way then Catholics. They celebrate the Xmas on the January 6th with Christians from many Orthodox country come to the main church ( Patriarchate ) where Patric ( head priest ) throw the cross to the Golden Horn and people jump in to the water to bring the cross back. " in January!!! "
However if you will be in Istanbul you will see lots of Xmas decoration in many district of Istanbul like Sultanahmet and Beyoglu - Taksim. Especially In Sulatanahmet it is almost every restaurant and hotel have a Xmas three at the door. That is to be aware of how important the Xmas is for Christians world and to welcome their guest and to make them feel at home as many Christians come to Istanbul in Xmas holiday.
So there will be Xmas decoration but there isn't a proper Xmas celebrations at homes like it happen in Christian world. I hope this will help you to understand about the Xmas time in Istanbul. almost 4 years ago