From Barcelona · 41.39° N · 2.17° E
Eight getaways for La Mercè: three plans to enjoy the main festival in the city and five outings, almost all by train, to places where 24 September is an ordinary day.
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On 24 September it is a holiday only in Barcelona, so in Cardona it is just another day and the castle has its usual visitors. If the getaway falls on a Monday, expect the castle and the Salt Mountain to be closed on Mondays that are not holidays there.

At the end of September the Penedès harvest is finishing its last weeks and the vines of the Camí del Vi are still laden. You get there on the direct R4, and in the Penedès the 24th is a working day: the cellars open on normal hours.

For those staying for the festival: an indoor workshop in Gràcia, away from the centre, while in Sant Jaume they raise castells. Park Güell entry is timed and on the days of La Mercè it sells out sooner: buy it in good time.

At the end of September the sun sets around half past seven, so the sailing boat leaves in the late afternoon and the Can Solé rice arrives at a normal hour. Seeing the city in festival from the water is worth the booking; Can Solé is closed on Mondays.

The Chocolate Museum is in the Born, a walk from the Plaça de Sant Jaume, where on the 24th the city's castells teams perform. Workshop in the afternoon and an early tablao: dinner at 18:00, with the show at 19:15, leaves time for the evening festival.

Sitges celebrates Santa Tecla on 23 September, its smaller main festival, with giants, castells and children in the leading role. The R2 Sud drops you there the day before La Mercè; the village museums are closed on Mondays.

The R4 train gets you out of the festival in fifty minutes, and in Sant Sadurní the 24th is a working day, with the cellars in full activity after the harvest. The underground train tour is indoors: it works if it rains too, which at the end of September it does.

At the end of September temperatures drop and the climb to Sant Jeroni becomes pleasant again. You get there without a car: R5 from Plaça d'Espanya to the Aeri station. If you want to hear the Escolania, check its calendar first: it does not sing on Saturdays or during its holidays.
Questions
It is the day of La Mercè, a local holiday in the city. At midday Barcelona's castells teams perform in the Plaça de Sant Jaume, and in the days around it there are concerts, correfocs and the piromusical.
No. Only in the city. In Sitges, the Penedès, Montserrat or Cardona it is a working day, with wineries and museums on their normal hours.
To Sitges and Garraf on the R2 Sud, to Sant Sadurní and Vilafranca on the R4 and to Montserrat on the Ferrocarrils R5 as far as the Aeri. All are an hour or less away.
The castells at Sant Jaume and the giants, and in between a cookie workshop at the Chocolate Museum. To get out, the train through the Codorníu cellars, where under-eights go free.
Geocaliahttps://www.geocalia.com/en/spain/catalonia/barcelona/getaways/merce-long-weekend