Planning for Controlled Demolition WTC 7, 1 & 2

So, given all that we know about WTC 7. Especially with the just released footage from BBC, and seeing that they reported on the collapse of the building 20 or so minutes before it actually did, I'm curious to learn when people think that the planting of explosives could have been done. I'm sure a project like this must take a lot of time. Given all the chaos of that day I highly doubt they were able to do it during the morning and afternoon before it fell at 5:20.

I remember watching or seeing a movie or clip that talked about a theory that the planting of explosives could have happened during the construction of WTC 1 & 2. Does anyone think this is possible or was it done in the weeks or months leading up to 9-11. I know there were days with major power outages in the buildings so it could have happened then.

What are your ideas on how and when they planted explosives in WTC 7 or WTC 1 & 2