Today is Rosh Chodesh (“Head of the Month”) for the month of Kislev.
Special portions are added to the daily prayers: Hallel (Psalms 113-118) is recited — in its “partial” form — following the Shacharit morning prayer, and the Yaaleh V’yavo prayer is added to the Amidah and to Grace After Meals; the additional Musaf prayer is said (when Rosh Chodesh is Shabbat, special additions are made to the Shabbat Musaf). Tachnun (confession of sins) and similar prayers are omitted.
Many have the custom to mark Rosh Chodesh with a festive meal and reduced work activity. The latter custom is prevalent amongst women, who have a special affinity with Rosh Chodesh — the month being the feminine aspect of the Jewish Calendar.