Looking so far a head is difficult in Spain. For example in August this year our local town festival had Milladoiro as the headlining group. Unfortunately this was not announced until a week or so before.
The website for Ortigueira has not been updated for 2019 yet. http://festivaldeortigueira.com/
Thank you for your reply. I will look at the link you shared. We have found a Wikipedia site with piping festivals. I will post more specific questions as we narrow down our dates and towns. We would like to spend three weeks in Galicia, perhaps some time in Asturia...
As I said the gaitas or bands that play gaitas are not always in festivals specifically for them. You can find them at most town fiestas. But which band and at what time will depend on a lot of things.
These bands can also be found in Asturias.