My husband and I have been avid RS travelers for years. Usually start by finding a local festival and build itinerary from there.
But we're now at a stand still, where's the next perfect one? A few we've loved are, Opening Octoberfest, Stammersdorf, Austria Wine Fest, Impruenta, Italy Grape Fest, Swiss Federal Yodel Fest, Estonia Song and Dance Fest, Tarragona, Spain Santa Tecla Fest, Baroque Fest in Porec, Croatia. We like those that specialize in the local traditions. Thank you for your suggestions!
Edinburgh Fringe Festival in August/September
Salzburg Festival?
Museums Riverbank Festival in Frankfurt, the last weekend in August. This year it is 23-25 August.
400 stands with all kinds of hand made items, lots of ethnic food as well as German, 14 music stages, dragon boat races, fire works, Organ Mile and Choir Mile in the downtown churches, special shows and workshops in all 24 of the museums and you get entry to all of them for all 3 days for usually 8€. If the weather is good, 2 million will show up, but because it is spread out so much, it doesn't feel so crowded.
Google Museumsuferfest videos to see.
Berlin's Carneval of Cultures happens every year at Pentecost weekend.
If you love sailing and water side festivals annual Kiel Week is the place to be.
Or spend Norway's constitution day May 17 in Bergen (full day), partly very traditional.
Not exactly a festival but Midsommar in Sweden.
Here is one for you. Takes place the week prior to Fat Tuesday: https://youtu.be/clf6a5zUSVQ Next year Fat Tuesday is 4 March so the festival will be a bit warmer.
How about Skye Live Festival. Focussed mainly but not exclusively on Scottish Traditional music. Held in Portree in May each year.
We just returned from https://www.calendimaggiodiassisi.com/
Highly recommended! It's held the first Wednesday to Saturday after May 1 in (of course) Assisi. The two parts of the town ("Sopra" and "Sotto") compete for the Palio of the town by reenactments of Renaissance activities. The town takes this very seriously-- it seems as much for their benefit as for a "tourist" event. The costumes and parades are worth the price of admission.
If people have not found it, there's a RS page just for fests: https://www.ricksteves.com/europe/festivals
Not all inclusive obviously, but worth a look.
Valencia Spain Fallas Festival.
I've been twice.
https://en.m.wikipedia.org/wiki/Valencia_Fallas
I would suggest Züri Fäscht, celebrated in Zurich every 3 years, but 2023 may have been the last. The city is apparently changing the format and looking for a new organizer.
https://www.myswitzerland.com/en-us/experiences/events/zueri-faescht-2023/
Focused on Traditional Irish music, Fleadh Nua, in Ennis, Ireland, in the spring.
Breton culture in northwestern France, in Quimper (aka Kemper), at the Festival de Cornouaille in July.
I don't think many festivals in Europe come close to Semana Santa in Andalucia, in terms of the passion, the colors, the music, the silence, and the pageantry. The streets fill with tradition and a sense of community that's both deep and quiet. Processions move through old streets, their every step drawing spectators into a centuries old heritage. The contrast between the marching bands and the silent moments makes the experience even more spiritual. The vivid robes, detailed golden floats, and candle lit faces add to the atmosphere, for many a life changing experience.
You can see for yourself - https://youtu.be/N-EzhFOI5XU?si=tiS5a0niEmR7Saq9