The Bicol region has a tropical climate with two seasons: wet (June to December, sometimes longer) and dry (January to May). For clear views of Mayon — the entire reason most visitors come — the dry season is essential. Even then, the volcano often disappears under cloud by mid-morning, so plan to be at viewpoints around sunrise.
Typhoon season runs from June to early December and Bicol sits in the most active corridor; expect disruption in the wettest months. The Magayon Festival, celebrated through May in nearby Legazpi, is a regional cultural event worth catching if you are in the area.