I'm sending a New Years card to someone. I've read that instead of the date I can use がだん to have it held until January first. Do I just write がだん 2017?

See a translation

> to have it held until January first
No. It should be ねんが in RED in the address side of the post card for the Japan post office to have it until New Year's morning delivery. The ねんが（年賀 in kanji） is short for ねんがじょう（年賀状） and it shows that "This is a New Years card". Write it in kanji（年賀） is better than in hiragana（ねんが） if you can. Remember, it should be:
1) Written in RED.
2) Written in the address side (I mean the surface which postage stamp to be sticked).

If you buy the official New Years card from the Japan post office, or buy a ready-made one from a book store/stationery store/convenience store in Japan, The 年賀 would be printed in red on it already.