Source: This summary is quoted from a thorough Wikipedia article which includes many citations

https://en.wikipedia.org/wiki/Day_of_the_Dead 

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"The Day of the Dead (Día de los Muertos) is widely observed in Mexico, where it largely developed, and is also observed in other places, especially by people of Mexican heritage. [...]

 

The multi-day holiday involves family and friends gathering to pay respects and to remember friends and family members who have died. These celebrations can take a humorous tone, as celebrants remember funny events and anecdotes about the departed. [...]

 

The celebration is not solely focused on the dead, as it is also common to give gifts to friends such as candy sugar skulls, to share traditional pan de muerto with family and friends, and to write light-hearted and often irreverent verses in the form of mock epitaphs dedicated to living friends and acquaintances, a literary form known as calaveras literarias. [...]

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Mexican academics are divided on whether the festivity has genuine indigenous pre-Hispanic roots or whether it is a 20th-century rebranded version of a Spanish tradition developed during the presidency of Lázaro Cárdenas to encourage Mexican nationalism through an "Aztec" identity." [...]

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Read UUA guidance about avoiding appropriation of this day.