This is the 6965th most frequent Spanish word.
díaz
"Díaz" is a Spanish surname.
In this sentence, "Díaz" is used as a proper noun, likely referring to a person's surname.
Díaz está trabajando en el proyecto de investigación.
Díaz is working on the research project.
Here, "días" is used as a plural noun meaning "days," referring to instances of daytime.
Los días soleados siempre me alegran.
Sunny days always cheer me up.
In this case, "Día" is a singular noun meaning "day," used as part of the phrase to denote a specific commemorative day.