Here's the essential part of the original post with an update for this year:
The same is not true even of 9 Av. Why is 10 Teves more significant than other fast days? Why should the beginning of the siege process that years later led to churban be more significant than the churban itself?One more point about this date: The 10th of Teves is a day marked for remembering tragedies, even those that are not recorded. In the State of Israel, this is the day designated for saying Kaddish (the Jewish prayer for the deceased) for people whose date of deaths has not been determined. Read more athttp://www.ou.org/chagim/roshchodesh/tevet/fast.htm
Chasam Sofer explains that 17 Tammuz is a fast which commemorates past events – the walls of Yerushalayim were breached. 9 Av is a fast which commemorates past events – thechurban, among other tragedies, took place. Same for the fast of Gedlaya. Not so the fast of 10 Teves. True, the siege was put in place on 10 Teves, but other enemies has also laid siege to Yerushalayim and they were defeated. There was time yet to avert a churban. The fast of 10 Teves is not a fast that commemorates events which already occurred, but is rather a fast of an eis tzarah, a fast to avert future tragedy.
The failure to rebuild the Mikdash is tantamount to witnessing its destruction. The din v’cheshbon[spiritual reckoning] of whether this year will be another year of continuedchurban or whether this year will be the year we avert 9 Av and witness the rebuilding of the Mikdash occurs on 10 Teves. The future is in our hands to determine.