Tradition tells us that Sant Medir was a Catalan peasant who lived in the early fourth century in the Collserola mountains, near what today would be Sant Cugat del Vallès, and who was beheaded by the Romans for supposedly helping a priest. The traditions and the worshipping of the saint were lost and almost completely forgotten until, starting in the 19th century, the residents of Barcelona's Gràcia and Sarria-Sant Gervasi neighbourhoods started picking them back up again. These areas are where the celebrations originated and are now held every year on March 3.
Saturday is Sant Medir's feast day, and it's celebrated with parades of 'colles' (groups who get together for the day's activities) from approximately 9am to 1pm in the neighbourhoods of Sarrià-Sant Gervasi, Gràcia and La Bordeta, regaling kids with loads of sweets. You'll find them in Major de Sarrià, Plaça del Consell de la Vila, Gran de Gràcia, Torrent de l’Olla, C/Escorial, Travessera de Gràcia, Plaça de la Vila and C/Constitució.
Generally the colles are made up of horse-drawn carriages and other vehicles led by a marching band to liven up the party. The so-called pilgrims in the parades throw sweets out to those lining the routes, often passing by city markets, squares and schools, where more people are likely to be, but also heading down narrow streets and wide avenues so you have to run after them for your prize.
Around noon to 1pm all the colles meet up at the Sant Medir Hermitage in Collserola. They celebrate with a Mass in honour of the festival's patron, and they all receive a commemorative ribbon.
When night falls it's time to celebrate Sant Medir in Gràcia, especially in C/Escorial, C/ San Salvador and Gran de Gràcia. The colles gather in formation from C/ Nil Fabra back. At 8pm the parade gets underway and heads down Gran de Gràcia led by the City Mounted Police, while onlookers await them and their boiled sweets along the route. At the end of the parade, in Pla de Salmeron a platform is set up where the local authorities wait to greet the colles one by one, often exchanging sweets.