It's when you eat.
Perhaps we should note that in the Athonite tradition the morning trapeza is informal, i.e. there is no procession from the Church after morning services to the trapeza. Thus, there are usually no corporate prayers (certainly not of the same type as those for the official trapeza), no readings during the meal, and only a select minority of the brotherhood actually attends the morning meal (usually consisting of tea, bread, honey, olives -- perhaps dates and halva).
The only official meal of the monastery, at which all monks eat, is the evening trapeza, which takes place after the Vespers. The brotherhood makes an official procession from the Church to the trapeza, has a set rite of pre-meal prayers, listens in silence to readings from the lives of the Saints during the meal, ends the meal in an official rite of prayer, and then processes back to the Church for Small Compline/Canon to the Theotokos.
Unless he receives special approval, a typical healthy monk only has one meal a day (the evening one).