Franz Josef has a temperate-rainforest climate — wet year-round, with around 5,000 mm of rainfall annually. Plan for rain whenever you visit, and consider any clear day a bonus.
December to February (Southern Hemisphere summer) is the busiest and warmest period, with temperatures in the high teens to low twenties Celsius and the longest daylight. Heli-hikes and scenic flights are most likely to run; accommodation needs to be booked well in advance.
March to May (autumn) and September to November (spring) are quieter and often clearer between rain bands. Temperatures drop into single figures at night. June to August is winter, with cool days and a slightly higher chance of snow on the ice — visually the glacier is at its best, but heli-flights are more often cancelled by weather.
Whenever you come, build a buffer day into your plans: at least one day of your stay is likely to be lost to weather, and you want flexibility to take the glacier flight on a clear morning.