The Haftarah of Shabbat Rosh Chodesh is read whenever Rosh Chodesh falls out on Shabbat, and overrides any other Haftarah. The only exception is Shabbat Shekalim and HaChodesh, which are tied to Rosh Chodesh Adar and Nissan, respectively.
It is not an easy Haftarah; it contains some very powerful imagery, both of comfort and of fear.
Linear annotated translation of the Haftarah of Shabbat Rosh Chodesh
The obvious connection is the repetition of the words “Shabbat” and “Chodesh”. Understanding why Shabbat and Chodesh are mentioned, and what they have to do with the general theme of Geula (redemption), now that is not obvious at all.