I was served a subpoena for a December 15th custody hearing date. I am friends with the wife , but the husbands attorney sent me the subpoena.
The wife (my friend) told me they are going to have to get a new hearing date because they were on standby and now there isn’t enough time in the
judges schedule to hear them that day. She said her ex’s attorney is supposed to notify me that I dont’ have to go.
My question is, when they get a new hearing date, will I have to be served all over again or does that original subpoena somehow carry over to
whatever date they pick in the future?

Yes, you will be served with a new subpoena for the new hearing date.