John Gill Commentary Luke 21:7

"And they asked him, saying, Teacher, when therefore shall these things be? and what [shall be] the sign when these things are about to come to pass?" — Luke 21:7 (ASV)

And they asked him. That is, his disciples, when they were come to the Mount of Olives, and as he sat upon that, (Matthew 24:3).

saying, master, but when shall these things be? when the temple shall be destroyed; and one stone shall not be left upon another;

and what sign [will there be] when these things shall come to pass? which shows that this refers to the destruction of the temple, and so the signs following; (See Gill on Matthew 24:3).