Can we have a cheat day? What if we need to attend a party or a function where we do not have a choice?
Even while we cheat, some food items such as meat, eggs, milk and its product should not be had as they have a devastating impact on our minds more than our bodies. If you're trying to heal a disease, no cheating should happen for the first 3 months. Every cheat meal will pull you 5 days back from the progress you had made.
Even when you cheat, choose the least damaging foods. Gradually, as you keep following this lifestyle, your mind will start to feel so much joy and contentment in pure Satvic foods, that you inherently won't desire cheating.