Not only dinner or lunch
A sagra is a local feast, sometimes it's "the" local feast and usually, quite always, some entertainment is scheduled, especially after dinner.
Traditionally, in many sagras a ballroom music concert starts around 9 pm or later in the evening. In Italian, this music is called "liscio" or "ballo liscio" and it's very popular among elders. Also disco music or other "modern" music concerts like rock, pop, reggae and so on, are also scheduled, albeit more seldom.
Other special events might be organized, such as historical parades, art or photos exhibitions, debates, games for children, crafts, food and artisans markets, bike races or other sports events. This really may vary from not entertainment at all to a giant village party, so there's not really a rule. Some sagras are intimate and quiet, most of them actually are noisy and chaotic. In addition, sometimes there might be some stands with charity offers.
You can have an idea about the festive sagras' atmosphere from our collection of videos, in Italian language.