Swiss meringue is a very stable meringue compared to the more commonly used French meringue.
Swiss meringue is a meringue that has been heated and cooked so that the sugar has melted and reached a certain temperature whereas the French meringue is a raw meringue.
I won't go into too much details about the preparation of the Swiss meringue but due to the cooked nature of Swiss meringue, it can be stored much longer than the French meringue. The cooking helps make the meringue denser which allows it to keep its form better. The cooking also provide the secondary effect of pasteurizing the egg mixture which also extend shelf life.
A Swiss meringue can be prepared a day in advance and should be stored in the refrigerator, covered. It will not leak as a traditional meringue typically do. It is not recommended to freeze it though.
Nonetheless is it still a meringue which is unstable at heart. Its structure will slowly deteriorate but it isn't too noticeable until 2+ days. And if stored properly in the refrigerator it should fall apart before it becomes unsafe to eat.