May 7, 1895
Above, the earliest reference I can find to Cinco de Mayo in The Times: A celebration in Tucson featuring the Philharmonic Band, otherwise known as the Club Filarmonico Tucsonense, led by Federico "Fred" Ronstadt. And, yes, that’s the same family as Linda Ronstadt.
Below, the earliest reference I can find to a Cinco de Mayo celebration in Los Angeles.
May 6, 1896