My grandfather was born in this small town about 30 minutes away from Pachuca. As you may know, most of their colonial traditions remain. They used to honor their deities and saints, and some towns got their names in honor of the saint they considered was watching and protecting them, so. every year. they will celebrate his "birthday" with a family fiesta full of folklore.
In San Mateo, the celebration starts early in the morning on September 21st when most of the locals will attend to church service (Catholic). After the service there's a festival where all the children between the ages of 4 to 13 will perform some dances like deer dance, rain dance, and some others with some fireworks to scare rain away to enjoy the party. Every single person will have a big lunch with barbacoa and pulque. If you get there, everybody will invite you to join them; right after lunch you can go to see bullfight or charreada.
At noon you can have dinner at any of the places they have any kind of Mexican food and beverages, including tamales, chalupas, tacos, pozole, pulque, tequila, cantaritos and cafe de olla - you name it. You better have a good dinner because you may need extra energy to keep aware all night for the celebration.
At 10pm the real party will start with a fiesta and a firework "castle" with all kind of colors and figures. At the end, the top of the "castle" will launch a little crown spinning in the air, displaying a rain of stars of all colors, noisy but precious!