I pay Zakaat every Ramadhaan. A few days ago, I received a substantial sum of money.

Answer

Even if money is received one day before expiry of the Zakaat year, Zakaat has to be paid on it as long as the money has not been spent. So whatever cash you have, Zakaat has to be paid on it even if you received it just recently. Since you are a Zakaat -payer, it will be said that one year has passed on every sum you have received during the course of the year, and which you still have on hand.