“Ya” is Japanese for “night” and “za” is Japanese for “to sit” or “sitting.” Yaza is nighttime sitting (in zazen/meditation).
We will typically find various vegetarian snacks on a kitchen counter at a sesshin where yaza is encouraged. I never fully understood that practice. It’s hard enough to continue sitting throughout the night after sitting all day when faced with the knowledge that another full day of sitting awaits. And snacking just makes me even more sleepy.
However, if a sesshin sitting day formally ends at 9:30 p.m. or so, we can try walking around for half and hour to get rid of the stiffness caused by sitting all day, and then maybe tack on one hour or so of yaza, avoiding the snacks. That’s my personal advice to myself.