There is no such thing as a festival forecast months ahead: weather services can see specific days only a few days out. But big events have a history, and that history tells you what to pack for.
What the site counts
The dates of past editions of events in different countries come from Wikidata and Wikipedia, and the weather for each has been calculated from the ERA5 reanalysis for 1991–2025. For the usual dates of the next edition you get the chances of rain, heat and cold nights. This is climate, not a forecast for any particular year (methodology).
How to read the numbers
A rainy day (1 mm or more), “mud” from 40 mm over the festival days and the week before them, a hot day from 30 °C, a cold night below 8 °C. Rain on one of the days calls for a raincoat, mud calls for wellies. For marathons the figures look at the temperature at 9:00, rain during race hours (8–14) and 20 °C or more by 14:00; for Christmas markets, evenings between 17:00 and 21:00: frost, wet evenings (0.5 mm or more), snowfall. The ERA5 grid of about 25 km records 5–7 points more light rain than weather stations do, and its daily maximum runs 1–2.5 °C lower.