Is zakat to be paid in installments whenever a year has passed on an amount, or is it to be collected on a single date, knowing that collecting it might lead to delaying or expediting zakat on some parts?

It is preferable for a Muslim to designate a specific month — such as Ramadan (definition) — for paying his Zakat (definition). He should assess his wealth to see if it has reached the nisab (definition), then pay Zakat on all of it. Any wealth acquired during the year should be added to the original amount and Zakat paid on it accordingly.