Yes, book ahead, as many Europeans, and Spaniards, take advantage of their time off to travel and walk the Caminos. Saw many people struggle to find a bed starting on Good Friday all the way to Easter Monday, bit no issues before that, or after.
Each village will have its celebration on different days of the week. You will see posters on the doors and walls of bars, grocery stores, etc. So it may be a bit difficult to plan to see some of the celebrations.
What is common are the parades of floats and penitants often preceeded by a brass band. A little weird for those of us who associate pointy hats and covered faces with past, but alas also current, white supremacist idiocy in the USA. (Surely that is something I can say even on this Forum?) These parades can take place during the day as well as late in the evening, which may keep you up for longer than a pilgrim may want to be up at.
This year I was in luck as I was in Merida on the day free torrijas were being served on the main plaza. Yummy! Torrijas are basically French toast from a baguette/bocadillo type bread, served with a honey or wine syrup.