Matcha is very delicate stuff. It doesn’t like heat, it doesn't like air, and it doesn’t like light. Once it is ground, the clock on its shelf life — about a year, under ideal conditions — starts ticking.
And that’s for unopened matcha green tea; once it’s opened, it really should be used up reasonably quickly, say within a few months, for optimum freshness, color, and taste. It will still be fine after that, especially if it's been refrigerated, but there's really no reason to “save it for a special occasion” — it's never going to get better, so it’s vastly preferable to enjoy it in its optimum state. Stale, dead matcha is no fun — drink it when it’s hypergreen and vibrant.
Matcha should either be stored in the freezer, if you plan on NOT opening it for a while, or in the fridge, once it’s opened. In both cases, use a container that’s as air-tight and light-tight as possible. Our tins were designed with this purpose in mind. Close the tin quickly once you’re done and return it to the fridge.
It’s really important to store it properly: it’s absolutely key to fostering the tea’s taste, color, umami content, frothability, and health properties.