"S Razh-dess-tvom"
* S: Like the English "s"
* Rozhdestvom: The "zh" sound is like the "s" in "measure." The "e" is short, like the "e" in "met." The "v" is voiced, like the "v" in "very."
You can also use the more formal "S Rozhdestva" (pronounced "S Razh-dess-vah").