Fish are opportunistic eaters and will go mad for food whenever it is offered - like a starving dog to a bone. This does not mean they are hungry. It means they are grabbing the food as quickly as they can while it is there as they don't know when the next lot will appear, nor if a predator is going to come along and eat them or the food.
My fish are not fed very often (about 2x a week. If daughter in law or youngest grandchild visit they get fed more often

) but my fish are mature, as is the tank, and lightly stocked with lots of plants. If you have young fish though it is important to feed them more often (at least daily if not more often. A little bit often is best) so they grow quickly and to their full potential without becoming stunted.
Not feeding your fish one day a week will not hurt them at all.
Not only is the amount important but so is the quality. By a good quality brand and do not buy large containers of food thinking it is more economical unless you will go through it quickly. As soon as a packet's seal is broken the food starts breaking down and all the vitamins etc start to deteriorate. I seem to remember 3 months being mentioned as the time frame fish food is at its best but I wouldn't quote me!