This is something I'd love to find... but I am not sure anyone has created a comprehensive, updated one (I imagine it would be a lot of work to maintain, even if created).
Major things tend to be promoted at tourist-regional level (the regional tourist boards do not all coincide with regions used for other purposes). The nationally-recognised tourism regions are listed here: https://www.visitportugal.com/en/destinos
Click on the region, then on the next page, select Find Out More, for the official links.
However, many of the most interesting things don't quite hit the national/regional agendas, so yes, if you're spending time in a particular place, google something like "[placename] turismo eventos" - some municipalities have the info on their own sites, which are usually cm-[municipality].pt but others have a separate tourism and/or culture page.
There are many rather outdated sites mentioning annual events in particular places - unless you see 2021 attached, don't make plans without checking it's on this year (and this would be true even without COVID, as some events run for a year, or a few years, and peter out, especially if they are of relatively recent creation.