Pilot season is a season where the bulk of the casting for broadcast networks happens. That’s largely the distinction. It starts in January and goes through April. Generally speaking, that’s the season.
The traditional broadcast networks: NBC, ABC, CBS, and FOX, sell their shows to advertising which goes to Upfronts, which happens at the end of the season. But we are also living in a time where the cable networks and streaming networks like, Hulu, NetFlix, Amazon, etc. are developing and casting pilots all season long.
Comparatively, summer is slower than the rest of the year. But with every year that passes, Show Business is becoming a year round industry. I see so many actors check out for the summer. They go home. Go to weddings. Decide to just take it easy for a while. Yes, we all need breaks. But there a series regular roles being cast every month of the year now.