The Arizona Humane Society Circle of friends will be spaying and neutering pets for free on Thursday.
Check-in for the service begins at 6 a.m., but shelter officials said many people get there early.
"People must arrive early," said Bretta Nelson with AHS. "In months past when we've done this, people have arrived as early as 1 a.m."
Nelson said AHS is offering the program to battle overpopulation.
Spaying and neutering pets can protect them from diseases while getting them snipped can control their behavior.