Spring and fall are the best times, but since you're doing small areas you can also try seeding in summer. Just make sure to keep the soil surface moist until the grass is a few inches tall. You might have to sprinkle the seeded area twice a day with water, especially if the weather is dry or windy. You don't have to water deeply, since the seeds are on the soil surface.
To keep the soil from drying out as quickly, you can loosely scatter a little straw over the seeded areas, or cover the seeds with thin layer of water-holding potting soil.Another option is to buy reseeding kits that include mats that have the seed right in them and hold water well. It's a more expensive option than grass seed, but saves you some effort.If those bald areas are in deep shade, they are going to keep coming back, because grass needs sun. If that is the case, you could plant a shade-loving groundcover, or just cover the area with mulch.