Today Spanish expression comes from the rural world. April and May are months when rain is extremely important for plants and fruit trees blossom in all their beauty. If it rains a lot in these months, the result is a good harvest enough for the next year. For this reason it’s so important rain in May.
Now it’s not difficult to think about the meaning of the expression “Me viene como agua de mayo“, that is to say that something comes just at the right moment and thus it makes someone happy.
“La paga extra de Navidad me viene como agua de Mayo”. During Christmas time people use to buy a lot of presents, they go out for lunch/dinner and so on… and if a person doesn’ have so much money it’s always a pleasure receive an extra salary that helps to incur heavy costs. So, when we receive extra money, we say that they come “como agua de mayo“.