Seasonal events are usually the most rewarding to plan around because they cannot be experienced any other time. The universal calendar runs roughly: spring blossom and flower festivals, whose exact dates shift with the weather each year, so build in buffer days; summer open-air music festivals and coastal celebrations; autumn harvest events, wine and food festivals in producing regions, which are often the most local and least touristy of all; and winter holiday markets, light festivals, and ski-season events where climate allows.
Religious festivals add another layer and frequently follow lunar calendars, meaning dates move annually, so always verify the current year's dates on official sources rather than old blog posts.
Two planning notes from experience: for weather-dependent events like blossoms or foliage, aim for the historical peak window but stay flexible within it, and for famous festivals book accommodation months out since prices multiply. The smaller regional editions of big festivals, a village harvest fair rather than the capital's showcase, often deliver the more memorable day.