Zakat is due immediately upon your Zakat anniversary. If you’re not sure when this is or you’re paying for the first time, simply work out the day you first owned wealth over and above the Nisab threshold (if you don’t know the exact date you can estimate). Once you’ve paid your Zakat once this then becomes your Zakat anniversary.
Zakat can be pre-paid before your Zakat anniversary. This is to encourage people to increase in worship and make the most of blessed times like Ramadan or the first ten days of Dhul Hijjah. If you do pay early, you still need to work out your Zakat due on your Zakat anniversary to make sure you’ve paid the right amount.