There are many visitors who can show up on your island each day.
Only a single vendor can visit your island each day. Some set up shop in the plaza, while others will wander around. There should be a vendor visiting every day - Saturday is always K.K. Slider (after you unlock him) and Sunday will be Daisy Mae. On Sundays, an additonal vendor may appear alongside Daisy Mae. Gulliver, although he doesn't sell anything, replaces that day's vendor.
Some visitors can show up on random days, regardless of what vendor is there that day.