You can reseed in spring or fall, depending on the weather, at the end of March or the end of September.
As it rains a lot at these times of year and the soil is already or still warm, the Lawn seed relatively quickly and requires little manual watering. You can also sow in August, for example, but then you will have to water more frequently.
This is because if the seed starts to germinate and is not watered sufficiently or is not watered at all, it will dry out and be ruined. However, reseeding in summer is less likely to produce good results, as the very high temperatures are extremely stressful, especially for young grasses, and some seeds will not survive. You can avoid this by sowing in spring or fall.