During the opening scene of BttF, Doc calls his own lab to tell Marty about the experiment he wants to show him that night.
From a transcript:
Marty: You know, Doc, you left your equipment on all week.
This sounds like neither Doc nor Marty have been to the lab recently. This is further substantiated with Marty's surprise when all the clocks start chiming.
Did Doc--presumably through time travel--know to call that specific day and time to talk to Marty, had he been calling all week looking for him, or was it luck that Marty decided to drop by when Doc happened to call?