The calendar above shades today by the Umm al-Qura calendar, from your own device's date, and marks Ramadan, the two Eids, the Day of Arafah and the Islamic New Year. Convert any date between the Hijri and Gregorian calendars beside it.
The Islamic calendar
The Hijri calendar counts years from the Prophet's migration to Madinah. It has twelve lunar months of 29 or 30 days, so a year is about 354 or 355 days and Islamic dates arrive about 11 days earlier each year in the Gregorian calendar. Ramadan moves through every season over roughly 33 years.
Why this calendar can differ from your mosque
A new month begins with the new crescent moon. Saudi Arabia's Umm al-Qura calendar, used here, sets the dates in advance by calculation. Many communities instead wait for a sighting of the moon, locally or abroad, which can make the start of a month one day later. For Ramadan and Eid, follow your local announcement.
Why the date matters for zakat
Zakat is due once your wealth has stayed at or above the nisab for a lunar year, so your zakat anniversary is a Hijri date. Many people note it as a date in Ramadan. You can convert it here and set a reminder in your normal calendar.