When are there jellyfish?
Jellyfish seasonality for 40 countries, from GBIF occurrence records — corrected for the fact that far more people are at the coast in summer.
Why most "jellyfish season" answers are really tourism data
Nobody counts jellyfish. People report the ones they meet, so a raw monthly count of sightings is largely a picture of when people are at the beach. Publishing that as a season would just be publishing the shape of the holiday calendar.
Every month here is instead divided by a control group — molluscs, recorded by the same observers on the same coast in the same months. What survives is the seasonality that is not explained by there being more people about. 1.0 means "recorded as often as expected"; 2.0 means twice as often.
There is rarely one season
In British waters the lion's mane peaks in July and August, while the Portuguese man o' war — a far worse sting — peaks in October and November, arriving on autumn gales after the swimming season has ended. Same coast, opposite seasons. So species are reported separately rather than merged into a single answer.
By country
Country is the finest resolution the data supports: below a few hundred records a twelve-way monthly split is noise. The figures are never comparable between countries either — a national total reflects how much recording that country has digitised, not how many jellyfish are in its water. Full method