<season> is given, it will show seasonal manga based on given season.
Otherwise it will show current seasonal manga.
If you want to see all available seasons,
Current seasonal manga is retrieved from https://github.com/mansuf/mangadex-downloader/blob/main/seasonal_manga_now.txt. If you think this is out of update, please open a issue here
Get current seasonal manga
Seasonal: Fall 2020 manga
mangadex-dl "seasonal:fall 2020"
Get all available seasons