There is a Water day once a month or every 29th day. It always follows a Vitamin day. The Water day must always be followed by a Protein day. Drink only water, coffee or tea (without any sugar or other supplements). If you cannot restrain from food you may have some vegetable soup, but without any salt or cooking oil.
Water days could be skipped. In such case the diet will continue for 88 instead of 91 days in such case. It depends on your personal choice to observe them or not. Consider your health status in advance and consult a doctor. A lot of people report dizziness and fatigue during the day, which follows the water day.
There are two options in case you decide to skip it. The first one is to skip all water days and stick to the schedule. The other one is to have water and vegetable soups (cooked neutral vegetables) without any spices. The second option is considered a lighter variation of the water day.
It is recommended to consider the options carefully and to choose the one, which corresponds to you and to your health status.
If you decide to observe the water days you should be aware that they are not meant for weight loss, but for body cleansing (detoxification), which means that even if you lose a lot of weight after a water day, you will most probably gain the bigger part of it during the next few days. This is considered normal and should not disappoint or demotivate you.