It's great to hear that your no mow grass section is looking good! By no mow grass I am assuming some sort of fescue. If that is the case, the best time to fertilize is late August through early October and the second best time is May through late June. If you decide to fertilize, it would likely benefit from a nitrogen application. That said, it's good practice to perform a soil test to determine the specific nutrient needs of your lawn. If you haven't done a soil test it would be beneficial to do so before applying any fertilizer. The results of the soil test will guide you on the type and amount of fertilizer to use for optimal growth and health of your no mow grass.

Regarding crabgrass, a higher mowing height and applying the proper amount of fertilizer will help reduce weed invasion. The best defense against weeds is a healthy lawn. Small patches of weeds, especially easy-to-pull annual ones such as crabgrass, can be removed by hand. If herbicides are needed, be sure to follow all label directions and apply the proper rate and timing of the year for best control.

Due to their bunch-type growth habit, there is less potential for excessive thatch problems with many fescues. If you do decide to aerate do so when the lawn is actively growing. The best time to do it in Minnesota is from mid-August through September.

Good Luck with your boulevard. It sounds like it is off to a good start!

Here is a link to getting a soil test:

https://soiltest.cfans.umn.edu/testing-services/lawn-garden

And two resources on no mow grass care:

https://extension.umn.edu/lawns-and-landscapes/planting-and-maintaining-fine-fescue-lawn#fertilizing-3238012

https://www.extension.purdue.edu/extmedia/TURF-13-W.pdf