It is not permissible to delay zakah until Ramadan

If the hijri year has passed on wealth that meets the nisaab (threshold for zakah), it is obligatory to pay zakah immediately; it is not permissible to delay it after it has become due if one is able to pay it. 

Allaah says (interpretation of the meaning): 

“And march forth [lit. hasten] in the way (which leads to) forgiveness from your Lord, and for Paradise as wide as the heavens and the earth, prepared for Al-Muttaqoon (the pious)”

[Aal ‘Imraan 3:133]

“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, prepared for those who believe in Allaah and His Messengers” [al-Hadeed 57:21] 

Moreover, if a person delays that, he does not know what will happen to him. He may die leaving this obligation unfulfilled; fulfilling one's duty is obligatory. 

Also the poor may be depending upon that, and if he delays it, the poor will remain needy and not have enough to meet their needs. See al-Sharh al-Mumti’, 6/197 

The Standing Committee was asked about a man who possesses the nisaab in Rajab but wants to delay paying zakah until Ramadan. The committee replied: 

It is obligatory for you to pay zakah in Rajab in the year following the year in which you took possession of the nisaab. But if you want to pay it in Ramadan of the year in which you took possession of the nisaab, paying it before the year has passed, that is permissible, if there is a valid need for it to be brought forward. But delaying it until the Ramadan that comes after the year has passed and it has become due in Rajab is not permissible, because it is obligatory to pay it as soon as it becomes due. 

Fatawa al-Lajnah, 9/392. 

Whoever is obliged to pay zakah and delays it with no valid shar’i excuse is sinning, because the evidence of the Quran and Sunnah shows that zakah must be paid as soon as it falls due. 

According to another fatwa (9/398): 

It is not permissible to delay paying zakah after the year has passed, unless that is for a valid shar’i excuse, such as there being no poor people at the time when the year has passed, or not being able to get the zakah to them, or because the money is not available, etc. But delaying it until Ramadan is not permissible unless the period is very short, such as if the year will have passed in the second half of Sha’baan, in which case there is nothing wrong with delaying it until Ramadan.