Saving the game state?

I want to be able to by pressing a certain key, the game will save everything inside of the world. Every mob entity's current position, the weather, the time, Everything. I then want to save that state and hold it, and then load it again anytime I want by pressing another key.

I have already setup the keybinding system, I just need to figure out the procedure to save the games current state, which includes the time, weather, and entity positions, including the players position, and then load it whenever I press another key. Does anyone know how to achieve that or something similar to that?

Please and thanks.