When is zakat obligatory: Is it immediately upon the money reaching the nisab, or after a full hawl (lunar year) has passed on it, and is it a condition to pay it on the same day of every Hijri year?

Zakat is not obligatory on money unless it reaches the nisab (minimum threshold) and a full hawl (one complete lunar year) has passed on it. Zakat must be paid on the day the hawl is completed, and it has no connection to the month of Ramadan or the beginning of the Hijri year. In fact, it is not permissible to delay it, waiting for Ramadan; rather, it must be paid immediately because it is a right of the poor.