Besides Chanukka, today we also celebrate Rosh Chodesh Tebet.
In the Tanakh (Hebrew Bible) the name of this month is Chodesh ha-Asiri, the tenth month, counting from Nisan. In the Torah the months are named numerically (first, second, etc). The name Tebet was coined in Babylonia, same as the other commonly used names of the Hebrew months (Nisan, Iyar, Cheshvan, etc.).
Some years Rosh Chodesh Tebet is observed for one day and some years--for example, this year--for two days. Why? Because Kislev, the preceding month, consists sometimes of 30 days (ma-le) and some years of only 29 days (chaser). The 30th day of the preceding month is always the first day of Rosh Chodesh of the next month, and the second day of Rosh Chodesh is the 1st day of the new month. Today, is the 30th day of Kislev, which is Rosh Chodesh Tebet.
The month of Tebet itself, is always 29 days long and because of this lack of variation in its length, Rosh Chodesh Shebat, the month which follows Tebet, will always be celebrated for just one day (the 1st of Shebat).
Today and tomorrow we say Ya'ale veYabo and 'al haNisim in the Amida and in Birkat haMazon.
In the morning we read the full Halel, then we take out two Sifre Torah. On the first one we read the Rosh Chodesh portion, but instead of dividing it into 4 parts (or Aliot) as we do every Rosh Chodesh, we divide the reading into 3 Aliot. In the second Sefer Torah we read the part corresponding to the 6th day of Chanuka. We also say Musaf, including 'al haNisim.