I 've heard that the celebration has changed a lot from what it used to be because it was meshed withCatholicism and other cultures add their own things to it, but how was it celebrated during the time of the Aztecs?
Well, the Aztecs did not really celebrate Day of the Dead. The Day of the Dead is a post-conquest Mexican holiday. While pre-Columbian cultures certainly had skeletal and death imagery, this imagery was not in a similar form or presented in a similar context as Day of the Dead imagery. And while Day of the Dead shares some certain characteristics with Spanish and other European traditions, such as offering food at cemeteries, it is not entirely a European holiday either. Instead, Day of the Dead was born during Mexico's colonial period in the face of rampant disease, tragedies, and death experienced by Natives, mestizos, and Europeans living in the country.
Brandes, Stanley. “Iconography in Mexico’s day of the dead: Origins and meaning.” Ethnohistory (1998): 181-218.