Praise be to Allah.
Firstly: When a full hijri year has passed and zakaah becomes due, it must be paid, unless it is the zakaah of agricultural crops, which must be paid on the day of harvesting, because Allaah says (interpretation of the meaning):
“but pay the due thereof (their Zakaah, according to Allaah’s Orders, 1/10th or 1/20th) on the day of their harvest”
Zakaah must be paid as soon as one full hijri year has passed, because Allaah says (interpretation of the meaning):
“Race with one another in hastening towards forgiveness from your Lord (Allaah), and Paradise the width whereof is as the width of the heaven and the earth”
Ibn Battaal said:
One should hasten to do good, for things change, death could come at any time, and delaying is not good.
Ibn Hajar said: Someone else added:
It is better in order to free oneself from blame and help others, more pleasing to the Lord and more likely to erase sin.
Fath al-Baari, 3/299.
Secondly: it is not permissible to delay paying zakaah after it has become due, unless one has a valid excuse.
Thirdly: it is permissible to pay zakaah before it is due, by way of hastening it.
Hastening zakaah means paying the zakaah of two years or less, before it is due.
It was narrated from ‘Ali that the Prophet (peace and blessings of Allaah be upon him) asked al-‘Abbaas to pay zakaah two years in advance.
(Narrated by Abu ‘Ubayd al-Qaasim ibn Sallaam in al-Amwaal, 1885. Al-Albaani said in al-Irwa’ (3/346): it is hasan)
According to another report:
It was narrated from ‘Ali that al-‘Abbaas asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about paying his zakaah before it was due, and he allowed him to do that. (Narrated by al-Tirmidhi, 673; Abu Dawood, 1624; Ibn Maajah, 1795; classed as saheeh by Shaykh Ahmad Shaakir in Tahqeeq al-Musnad, 822).
Fourthly: Giving charity to people is better in Ramadaan than in any other month.
It was narrated that Ibn ‘Abbaas said: The Messenger of Allaah (peace and blessings of Allaah be upon him) was the most generous of people, and he was at his most generous during Ramadaan, when Jibreel met him. Jibreel used to meet with him every night and teach him the Qur’aan. The Messenger of Allaah (peace and blessings of Allaah be upon him) was the most generous person, even more generous than the strong uncontrollable wind (in readiness and haste to do charitable deeds).” (Narrated by al-Bukhaari, 6; Muslim, 2308).
Al-Nawawi said: This hadeeth teaches us a number of things, including the fact that it is mustahabb to be generous during Ramadaan.
So if a person’s zakaah is due in Ramadan, or it is due after Ramadaan but he pays it in advance during Ramadaan in order to make the most of the virtue of paying it in Ramadaan, there is nothing wrong with that. But if his zakaah is due before Ramadaan (for example, in Rajab), and he delays it so he can pay it in Ramadaan, this is not permissible, because it is not permissible to delay zakaah unless one has a valid excuse.
Fifthly: There may be some reasons why paying zakaah at a time other than Ramadan is preferable to paying it during Ramadaan, such as if there is a major disaster or famine in some Muslim country, then paying zakaah at that time is preferable to paying it in Ramadaan. Another example is if many people pay their zakaah during Ramadaan to meet the needs of the poor, then the poor have no one to feed them at times other than Ramadaan; in this case paying it at a time other than Ramadaan is preferable, even if that leads to delaying zakaah, out of consideration for the needs of the poor.
Shaykh Ibn ‘Uthyameen (may Allaah have mercy on him) said:
It is permissible to delay zakaah if that is in the interests of the poor and will not cause them harm. For example, in our country many people pay zakaah in Ramadaan and this gives the poor what they need or more than they need, but then during the winter, if that does not coincide with Ramadaan, they will be in greater need and there are few people paying zakaah. In that case it is permissible to delay zakaah because that is in the interests of those who are entitled to it.
Al-Sharh al-Mumti’, 6/189
And Allaah knows best.