Whale-shark season in Donsol generally runs from November to May or June, with the best odds usually from February to April. Even then, this is wild animal watching, so build in more than one morning if the encounter is important to you.
Bicol is exposed to heavy rain and typhoons, especially from June to November. Outside whale-shark season, Donsol is much quieter and the firefly cruises still have appeal, but most travellers will not make the detour for that alone.