When should they make Yerushalayim rejoice?
Radak: In Galus, all its lovers should do so. This is like "Ki Ratzu Avadecha Es Avaneha" (Tehilim 102:15). When the exiles will gather, they will rejoice in it. They will see its building. Just like you saw its Churban and mourned over it, so you will rejoice in it.
Malbim: This corresponds to "Kol Yotzei me'Heichal" (6). It will rejoice Yerushalayim and all its lovers.
Why does it say "Gilah Vah"?
Malbim: There is a difference between Gil and Masos. Simchah and Gil refer to Simchah in the heart. Simchah is constant, and Gil is sudden Simchah when a person gets a good tiding. Yerushalayim's lovers will have constant Simchah due to its success. They will have Gil from time to time over new miracles that will be done in it. Sason teaches about external actions that people do to show their Simchah, e.g. wearing clothes of Simchah, Masos of drums and harps.
What is the significance of "Kol ha'Mis'abelim Aleha"?
Malbim: They made external signs to show mourning, like wearing sackcloth and ashes. Now they will make external signs to show Simchah, e.g. "Shemen Sason Tachas Evel" (61:3). This is Midah k'Neged Midah!
