Day of the Dead is most strongly associated with Mexico, where the tradition largely developed.
Día de los Muertos is a Mexican observance held primarily on November 1 and 2, alongside the Catholic dates of All Saints’ Day and All Souls’ Day. Families remember deceased loved ones by creating ofrendas, visiting cemeteries, sharing food, and using marigolds, candles, photographs, and personal belongings to welcome the dead.
Its history is layered rather than attributable to one single ancient festival. Indigenous Mesoamerican practices of honoring ancestors blended with Catholic observances introduced during Spanish colonization. Mexican communities shaped that combination into the distinctive traditions recognized today, including pan de muerto, sugar skulls, papel picado, and calaveras literarias.
The holiday is also celebrated by Mexican communities around the world, and related remembrance traditions exist elsewhere in Latin America. That international spread does not change the canonical answer: Mexico is the country most closely associated with Día de los Muertos and the place where its modern cultural form largely developed.