1)
From when are the years counted?
1.
Rashi: From Tzidkiyah's reign. Radak ? this was the year of the Churban. This is when Tzur said about Yerushalayim "He'ach".
2)
Which month was it?
1.
Radak: The verse did not specify. It seems that it was Av, the month of the Churban.
2.
Malbim: The verse did not specify. It seems that it was Elul or Tishrei after the Churban; they rejoiced over the Churban.
