Third tuesday in September.
On "Prinsjesdag" (very rough translation, "Day of Princes") the queen (or king) of the Netherlands holds the "troonrede" ("thronereason", ack. another bad translation) in which she tells the "Staten-Generaal" (the democratically elected leaders) about the current state of the Netherlands, what has happened in the past year, and what is planned for next year.
After the "troonrede", the "miljoennota" will be presented by the minister of finance. The "miljoennota" holds the balance for the past year. Also, he will present the financial planning for the next year.
Besides from the practical stuff, the queen parades from palace Noordeinde to The Hague in a gold-coated coach. And all the female politicians wear the weirdest hats they can find.