Here is what I plan to do based on an actual seeding date of September 9 (that answers your first question - mid-sept is the best time to seed in the north/transition zone):

1. buy some quality seed based on NTEP ratings for your area (www.NTEP.org)
2. If you have an irrigation system, perfect. I am jealous. If not, get a good set of timers and a good set of sprinklers that will be able to water your new seeds 3 times a day for 10-15 minutes per cycle.
4. call your nearest equipment rental place and reserve a verticutter or slit-seeder. And find a buddy with a lawn roller.
3. keep mowing and taking care of your weed patch for the next few weeks. Their time is short... so don't get too depressed.
^^^ these are things you can do now.
4. In mid sept, mow your lawn/weeds VERY low... like 1" - 1.5". Scalp the lawn basically. Bag the clippings.
5. use the slit seeder or verticutter to go over the lawn twice in different directions (i like 45 degree angles) with NO seed in the hopper.
6. run over the mess that gets chruned up with the mower and bag it up. If you are into hard labour, rake it up and bag it. Just get the debris off the lawn.
7. if you are using a slit seeder, fill the hopper, set it to the appropriate rate (1/2 the rate since you are making two passes here) and make two passes (not the same directions you made on the DRY run). If you are using a verticutter, then skip this step and spread the seed with a broadcast spreader. Again, use two passes at 1/2 the rate. this will help avoid 'cornrows'.
8. Apply starter fertilizer - I like Lesco - at the recommended rate.
9. Roll the seeds and fert into the furrows made by the seeder/verticutter. Use maybe 1/2 full of water.... no need to compact the soil - you just want to ensure good seed/soil contact.
10. Water pretty deeply to get the seeds further into the soil and give them a good start with moisture. I like to run each sprinkler for about 1 hour. for me that's maybe 1/2" of water.
11. Set your sprinkler timers to go off 3 times a day so the seeds don't dry out. I do mine at 10AM, 2PM and 4PM for about 15 minutes per run. The goal here is to keep the seeds MOIST at ALL TIMES! Once they get wet, don't let them dry out. You don't want puddles... Just want to keep the damp.
12. For TTTF you will see germination after about a week to two weeks. Longer for KBG.
13. Once the majority of the seeds have germinated, cut back on the watering, but water for longer. Like twice a week for an hour. Depends on your sprinklers, but the goal at this stage is to get water down deep so the fledgling grasslings will go down and get the water - establishing deep roots is the goal.

Whew. Ok. There are other nuances here and there that I have probably left out, but that is what I do for overseeding and I have always had success. The magic three components to getting your seeds to germ are fertilizer, soil and water - so get good seed/soil contact at the right ground temps (mid-sept), fertilize with a good starter fert and water water water to keep them moist. You should have success!!!

I'm not one of the resident experts, but can give you some knowledgeable guidance.

I'm in Northern Ohio and we are nearing the perfect time to overseed. I'm not sure how much colder you are then us, but I'd imagine now would probably be good for you up there.

Before overseeding you need to prep to have as much soil to seed contact as possible. Depending on the condition of your lawn, de-thatching and aerating are a good idea. De-thatching removes the "barrier" of old grown that is on top of the soil. Aerating also helps with thatch, decompaction, and watering. After one or both of those steps are complete you want to cut your current lawn short. Some people hack it all down in one cutting, others will cut say every other day until it is very short, almost scalped. This provides the seed the ability to make it to the soil, get light, and also keep your current grass under control while the new stuff germinates and grows before having to cut it and damage the new seedling.

Depending on the size of the area being overseed, you can spread by hand (Small area) broadcast spreader, or rent a slit seeder. Most commonly is to use a broadcast/fertilizer spreader. The slit seeeder is a more professional way of doing it as it places the seed just under the dirt to guarantee soil to seed.

After seeding it is a good idea to spread a starter fertilizer. Follow package instructions, or some recommend use half, and then doing the other half a few weeks later.

WATER WATER WATER. If the seed dries, it's gone. You must water 3-4 times a day for short periods just to keep it moist. As it sprouts you will slowly lay off and do once a day for longer periods and so on.

Some recommend top dressing with a compost or laying straw for water retention and protection, others say not needed. Your call.

Grasses are debatable and depend on sun and shade. There are many threads here to research further. Kentucky blue grass, Turf Type Fescues, and Perennial Rye, or a combination of all or some of them are going to be your choices.

...that should get you going. Good luck.