Tuvalu has a warm, humid tropical climate year-round, with sea temperatures hovering around 29–30°C and air temperatures between 26°C at night and 32°C during the day. There is not a bad time to be here in terms of temperature — it is always warm.
The main consideration is rain and cyclones. The wet season runs from November to April, bringing heavier rainfall and the risk of cyclones. Cyclones are most likely between December and March. Given how low-lying the islands are, even a cyclone passing at some distance can cause significant flooding and storm surge. Avoid these months if possible.
The dry season from May to October is the recommended time to visit. July to October gives the most reliably settled weather — less humidity, more sun, and lower seas for boat trips. This is when Funafuti's lagoon is at its most inviting.
There is no particular festival calendar to time a visit around, though the Tuvalu Day national holiday on 1 October marks independence and involves community celebrations, traditional dance, and sports on the main maneapa ground — worth seeing if you are there.
One practical note: flight schedules to Funafuti are limited and do not adjust seasonally. Whatever time of year you go, you are working around three weekly Fiji Airways departures from Nadi, so flexibility in your broader itinerary is useful.