A handful of food trucks do convene at the site of the Downtown Phoenix Public Market every Friday at lunch time (11am-1:30pm). For more details, you can check out the Phoenix Street Food Coalition's Facebook page, Twitter feed, or website (links provided below).
https://www.facebook.com/phxstreetfood (easiest channel to follow for updates)
http://twitter.com/#!/phxstreetfood (less easy to follow than Facebook feed)
http://phxstreetfood.org/ (infrequently updated)
Downtown Phoenix Public Market
721 N Central Ave, Phoenix, AZ 85004