Question
QUESTION: Fall is the time to sow grass seed in sparse spots, apply weed-killer (three-way, against  broad-leaf weeds), and fertilize.

But would you kindly advise me more specifically when, and in what order, to do these three things?  I'm in the Toronto area.

Thanks, Ronald.

ANSWER:  Clean and square out the area, sow grass seed, fertilize but only after the seeds have germinated and taken root . Discontinue any extra watering.  Apply weed killer only after you see weeds. Under no circumstances should you apply Trimec® before you are quite satisfied that you have got a satisfactory germination rate and the seedlings are firmly rooted in; because one of the chemicals in Trimec® will inhibit seed germination.  

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QUESTION: I guess you sent your helpful answer three times to make sure it transmitted.

Not sure what "square out" means. And I forgot to ask about the final mowing before the snow. I think that should be a bit higher than normal, but I don't know if weeds are more susceptible when they're high or fresh-mown.  Thanks again; I'm sure you're very busy.

Answer
I use the term "square out" to mean finishing the area to a rectangular shape that will accommodate whole pieces of sod. Narrow strips and small pieces of sod jammed in to complete an area are likely to dry out and die.
I have no experience with snow but I know that more leaf area means more "factory capacity" to manufacture more carbohydrates which the plant will store up and have available when spring comes around.
The same is applicable to weed killers e.g. Roundup?that are taken up through the leaves. The more leaf area, the better the uptake. The process is more effective on perennial rather than on annual weeds.