The day the Israelites stood at the foot of Mount Sinai, preparing to encounter God, is understood in midrash as the day of marriage between God and the Jewish people.
Events on the 17th of Tammuz, and others that also have been ascribed to this date, are the reasons it has long been observed as a day of mourning in Jewish tradition. Though the day and the rituals of this time of year are not often marked by Reform Jews, traditionally, the 17th of Tammuz begins the “Three Weeks,” a period of mourning lasting until the 9th of Av.
Rosh Chodesh, like many things in Judaism, has three hands.
On one hand, Rosh Chodesh is a simple acknowledgement of the visible stuff of the cosmos. As we know from Genesis Chapter 1–or just by looking out the window-we have the sun to shine by day, the moon