Where in Northern MI are you? Some areas stay cool enough during the summer that you can seed now and make it through. Other areas get hot enough that much of the grass will die and you'll need to reseed in the fall.

I live in Traverse City.

Brief tutorial.
Most grass will not grow in significantly shady areas. Around your trees might be too shady. North side of trees perchance?

You can try seeding but know this: crabgrass seed is looking for moisture right now. If you seed regular grass you will get crabgrass, too. If you can tolerate this until the end of summer (which might be right after the 4th of July for you), then you can seed safely for next season. Do you know what grass you have now (fescue or Kentucky bluegrass)? Fescue is somewhat shade tolerant. KBG is not.

Here are the basics of lawn care.

2. Water deeply and infrequently. Deeply means at least an hour in every zone, all at once. Infrequently means monthly during the cool months and no more than weekly during the hottest part of summer. If your grass looks dry before the month/week is up, water longer next time. Deep watering grows deep, drought resistant roots. Infrequent watering allows the top layer of soil to dry completely which kills off many shallow rooted weeds.

3. Mulch mow at the highest setting on your mower. Most grasses are the most dense when mowed tall. Bermuda, centipede, and bent grasses are the most dense when mowed at the lowest setting on your mower. Dense grass shades out weeds and uses less water when tall. Dense grass feeds the deep roots you're developing in 1 above.

4. Fertilize regularly. I fertilize 4 times per year using organic fertilizer. Which fertilizer you use is much less important than numbers 1 and 2 above.
   

I wouldn't call the areas significantly shady. The trees are in rows that run east-west on both sides of the house. They are between 6-7 ft high and there are 10 in each row. It is becoming bare on both the north and south sides of the trees. A bare spot is also to the southwest side of a single pine in the front yard. That one is about 12 feet high. The only other trees on in the yard are a couple of very small maples.

I have no idea what kind of grass I have now. It is almost if I have 2 completely different kinds. The lush thick kind around the house and a more wirey thin kind away from the house. How would one figure out what kind of grass they have?

I can tolerate it until after the 4th. My wife on the other hand...:) So seeding in maybe late July would be my best bet?