The first thing that will help you understand the title of this post is this fact: the city is in a desert.
Deserts do not produce much water.
Building a city with a couple of million people living in it comfortably requires good water management.
The solution this city has implemented is controlling the flow of the water pipes. One day each week the pipes to our neighborhood have the water flowing through them. Each household has water tanks on the roofs of their building to store water that you can use for the rest of your weekly water needs after the pipes are cut off again. Because you don't want to waste the water for the rest of your week, you try to accomplish all the big water activities on the day the pipes are flowing. For us, those big tasks are:
-watering the trees that line our garden.
-mopping all the tiles floors
-cleaning the toilets and tubs
-doing myriads of loads of laundry
Now, factor in being at school from 7-1pm and it creates a very busy day for me on Wednesday if I plan on using the flowing water. When we first moved in, I was very careful with using water for dishes, showers, etc. We've slowly been using more water during the week and haven't run out before water day, so that's good news. We've been told we have 3 cubic meters of water in our tanks every week. I've never had to think about my water usage before, so I don't yet know our average weekly water consumption.
It's very new to me to be in a place where water matters so much. The rights to water and the usage of water is of vital importance for this small country and this household.
Does that give you a good idea of what Water day is? Please, ask questions if you need clarification.