You may have heard a famous saying in the gym: "You should train the abs every day to get the best results." While you can train them every day if you want, this is not the best way to transform them because every muscle on the body is the same as far as its' comprised makeup.
Every muscle is made up of two basic types of fibers: Type I and Type II. Type I, also known as slow twitch, is responsible for functional endurance, and Type II is responsible for growth, power, and mass.
Distribution of these fibers in particular muscles throughout the body is partly genetic, and partly biological. Some people are born and naturally made up of more slow twitch fibers, while others genetically have more fast twitch distribution.
However, what sports you played as a youngster, teenager, and while growing up could play a role in the development of these fibers as well. If you were more of a runner or track and field athlete, your body may have grew and developed more slow twitch muscle fibers. If you were more of an explosive power athlete or did a lot of lifting and physical work growing up, you might have developed more fast twitch muscle fiber.
In either case, muscle fiber is the same in that in order to develop more of it and tone it up, the muscle fiber must first tear, or break down, during physical activity, and then grow back in adaptive response to that stimuli. This process takes at least 48-72 hours in most cases when done properly.
The abs are no exception. If you train the abdominals appropriately, causing the microscopic tears to take place that are necessary for growth and transformation, then you need at least 2-3 days for them to heal, recover, and transform.
Even if it is argued that the abs contain more slow twitch fiber than fast twitch, this is still not a valid argument because even slow twitch fibers need adequate rest time to recover. Like every muscle in the body, over training or training them every day or on consecutive days is counterproductive. The muscle needs to heal fully and grow back in order to transform.
Furthermore, if your abs are not sore on the day after you train them, then the fact is that you probably didn't train them hard enough or well enough to transform them. Like any muscle, the true test as to whether you have worked them well enough can be measured by how sore you feel on the days that follow. The more soreness, the more muscle fiber that got recruited and that broke down. In a similar fashion, the less sore you are, the less well enough you trained them and recruited the muscle fiber necessary to transform them and get results.
Even the calf muscles, which are notorious for being overwhelmingly comprised of slow twitch muscle fiber need adequate 2-3 days rest in order to transform, strengthen, repair, and rebuild.
To get the best results for abs, you must take the correct approach. Break down the fibers through intense training and progressive, adaptive training techniques, and then allow for the muscles to fully heal and recuperate before training them again, which requires at least 48-72 hours.