La Mercè and Barcelona's Christmas markets

La Mercè, around 24 September

La Mercè is Barcelona's own festival, held for the city's patron — the Virgin of Mercy — and it is the biggest thing in the Barcelona calendar. The date is fixed but the programme runs for several days around it, mostly in the old city and mostly free.

What it actually looks like on the ground:

• Correfoc. Fire runs: drummers and devils with sparklers and fireworks processing through the streets. Loud, hot, genuinely spectacular, and not for small children or anyone in flammable clothing.
• Castells. Human towers, built by colles in the squares, and the moment a small child climbs over the top of a tower of adults is the image of the festival.
• Gegants. Giant figures, some of them centuries old, danced through the streets with a band.
• Sardana. Catalonia's circle dance, performed in the squares by people of every age.
• Concerts and the closing display. Free music across the city and a large fireworks and music show by the water that finishes the whole thing.

It happens across the city, but the centre of gravity is exactly where you are: Plaça Sant Jaume, the Cathedral square, Plaça del Rei, the streets around them, and the waterfront for the finale.

The practical consequence: hotels in the quarter fill and rates rise for the whole week, not just the day. If your trip is fixed to late September, book months ahead. If it is not fixed, this is the week to avoid — or the week to come for, if a city on its feet is what you want.

The Christmas markets

Barcelona's Christmas is a market and a log. Both are worth knowing about before you arrive.

Fira de Santa Llúcia

The main market sits in the square in front of Barcelona Cathedral — which means, if you are staying in the quarter, it is your local. It opens in late November and runs to 23 December, with stalls selling nativity figures and the pieces of a pessebre: the scenery, the animals, the shepherds, and the figure every Catalan nativity has and no other does.

That figure is the caganer, and the stalls compete on it. If you only buy one thing in Barcelona at Christmas, buy that, because it explains more about Catalan humour than any guidebook will. You will also find the tió de Nadal, a painted log with a face and a blanket, which is beaten with sticks on Christmas Eve until it produces sweets. This is not a joke. It is the tradition.

Fira de Reis, on Gran Via

After Christmas, the market moves to Gran Via for the run-up to Three Kings' Day on 6 January: toys, sweets, and the tortell de Reis, the ring-shaped cake with a bean and a small king hidden inside. The two things you find in a slice decide whether you pay for the cake or wear a paper crown for the rest of the day.

The Three Kings' parade, 5 January

The Cavalcada de Reis is the biggest night of the Barcelona Christmas period, and it belongs to children. The three kings arrive by sea at the port and process through the city, throwing sweets, with floats and their pages. If you are in the quarter on 5 January, you are already on the route's doorstep — the harbour and the streets inland from it are the place to stand.

Other dates that fill the city

• Sant Jordi, 23 April. Catalonia's day of books and roses. Half the city is out buying both and La Rambla is impassable in the best way. Hotel demand spikes for a single weekday.
• Mobile World Congress, late February or early March. A week when every business hotel in the city is full and rates are unrecognisable. Avoid it unless you are attending it.
• Music festivals in June, and Formula 1 weekends. Both move through the calendar, and both make a large city feel booked out.
• New Year's Eve. Dinner is booked months ahead, and the street party happens in the squares rather than the clubs.

How to plan around them

The pattern is always the same: the person who plans the trip around the dates pays far more than the person who plans the dates around the trip. If any of the dates above are on your shortlist, buy the room first and everything else afterwards. Our quietest hotels page matters more than usual during La Mercè, and the under-€150 page is where to look for the rest of the year.