Some people may feel the 3rd, 4th, and 5th nicotine-free days are challenging because 3-5 days after you stop smoking, almost all nicotine is out of your body so your physical cravings peak and then subside.
After 5 days of being nicotine-free, almost all nicotine is out of your body and after that point, all cravings are mental. This video explains more: https://fb.watch/3r_7dx8rjh/