The Special Charm is a wallet item that greatly increases your daily luck. To obtain the Special Charm, you must first collect Secret Note #20. After that, head over to the truck next to JojaMart and give the driver a Rabbit’s Foot to permanently unlock the item.
This item has been made available through the 1.3 Beta update, however it could previously only be obtained with the use of a Save Editor.
In short, when you have the Special Charm, you have a very high chance to have to highest daily luck possible. This is exactly how your daily luck is calculated:
If you do not have a Special Charm, your daily luck is set to a random number between -0.1 and 0.1. If you do have the Special Charm, the game will roll a random number between 0.0 and 1.0 – if the number rolled is less than 0.8, your daily luck will be set to the highest amount (1.0).