If you're not going to be playing on a certain day, you should still spend your Mist Energy.
The easiest way to do this is to simply craft items.
Whether it's a 1, 2 or 3 star item, the Mist Energy doesn't cost anything to replace.
You can get ten 1 star items, so that's ten Haze bombs, for free. Ten chances of getting an epic Unique Variant and ten items you could potentially sell on the Bazaar for monies.
Or two 2 star items, so you could make two Autoguns, or two Caliburs. It would be best to combine them if you have neither.