Said Rav Gidal, "Rav said: When Rosh Hodesh occurs on Shabbat, the reader of the haftarah need not cite Rosh Hodesh [in the liturgy], for was it not Shabbat, we would not read from the Prophets on Rosh Hodesh."
Our liturgy is a reflection of our values. On Shabbat, festivals, and Rosh Hodesh (the new moon), we read publicly from the Torah to connect our souls more deeply with God, divine wisdom, and the mitzvot. We also read from the Torah on Mondays and Thursdays so that we never go more than three days without Torah. However, on Shabbat and festivals we read a much larger passage than on Rosh Hodesh. Why? On Shabbat and festivals, when gainful labor is prohibited, we do not go to work after our tefillot (services). On these days, we have the time to sit and hear the words of the Torah at our leisure. Similarly, we have time to hear a public reading from the Prophets, the haftarah. On Rosh Hodesh, we do go to work after tefillot and do not have the time for either a long Torah reading or a haftarah. We never read a haftarah if the day is only a Rosh Hodesh.
Now if it happens that the new moon falls on Shabbat, do we mention that salient liturgical fact in the haftarah blessings? Rav tells us that we do not. The haftarah is read only for the sake of Shabbat. Were it only Rosh Hodesh, we would not read it. Two thoughts occur to me: first, Shabbat has a value in our system that exceeds the regular cycle of the calendar—no matter the time of year or the date, Shabbat returns continually every seven days. Second, we seem to study the Prophets on Shabbat because we have the time to do so. Shabbat is a time for us to stretch our souls and study things that may be new to us.