Today is the second of the two Rosh Chodesh ("Head of the Month") days for the month of Adar I (this year being a leap year there are two months called "Adar" -- Adar I and Adar II).
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
This year is a shanah meuberet (lit., "a pregnant year") or a leap year on the Jewish calendar. The Jewish leap year, which occurs 7 times in a 19-year cycle, has 13 months instead of the regular year's 12. This is so that the lunar-based Jewish year should remain aligned with the solar seasons (12 lunar months make up a total of 354 days -- slightly more than 11 days short of the 365.25 day solar cycle). The added month is called "Adar I" and is inserted before the month of Adar (termed "Adar II" in leap years).
The festival of Purim celebrated on Adar 14, is in Adar II on leap years, while the 14th of the Adar I is marked only as "Purim Minor." Similarly, birthdays and most other anniversaries are marked on the 2nd Adar.
When the Rebbe accepted the mantle of leadership, this was his “acceptance speech”:
If you find a Jew who has love of G‑d, but lacks love of his people and love of Torah, tell him that this love cannot last.
If you find a Jew who has love of his people, but lacks love of G‑d and love of Torah, work with him to nurture this love until it overflows into the other two, until all three join in one tight knot that will never be untied.