After a low tide or storm are best along with the months of January, February, and March. On a regular day,  the a.m. low tide will be better than the p.m. because if you're the first one out in the morning, you'll be able to find the shells that have freshly washed ashore. Tip: My family has always gone to Sanibel Island in June and the beaches have always been filled with shells. There are shells all year long.