A Zakat anniversary is just like any other anniversary in the sense that it is a fixed date in the calendar to mark a significant date or milestone. More specifically, a Zakat anniversary is marked in the Islamic calendar as opposed to the Gregorian calendar.
What difference does that make?
If your Zakat anniversary is on the 1st Ramadan for example, this year that would have meant your Zakat anniversary was on 6th June 2016. Last year it would have meant your Zakat anniversary date was on 18th June 2015. So if you had stuck to the Gregorian calendar, you would have missed your actual Zakat anniversary by 12 days. If you are married and your spouse normally expects you to mark your wedding anniversary, imagine how they would feel if you remembered not just one day later but almost two weeks later?!
Zakat is much more significant than an anniversary; it is an obligation and pillar of Islam, so knowing when your Zakat anniversary falls is essential. Being late with Zakat means being late with an act of worship and being late in giving the poor their due. On the day of your Zakat anniversary, the portion of wealth which you owe in alms no longer belongs to you, even if you are still in possession of it. To hold onto it intentionally without a valid reason therefore is not permitted in Islam. The wealth we have after all, is simply that which Allah Almighty has entrusted us with as a test and opportunity to reap His rewards.
So, how do you know when your Zakat anniversary is?
Every believer will have their own Zakat anniversary. To work out your date, it is exactly one lunar year after the date when your Zakatable wealth first reached or exceeded the value of the Nisab. If you cannot remember the date your wealth first reached that level, then the date should be estimated. If this is not possible, then a specific Islamic date should be selected arbitrarily and adhered to annually going forward. Paying Zakat in the month of Ramadan is not necessary and every believer will have a different Zakat anniversary or due date, although giving charity in this month guarantees greater rewards if you cannot remember or estimate when you first met the Nisab .