Zakaat al-Fitr is an individual obligation upon every Muslim who can afford it, prescribed by the Shari‘ah (Islamic legislation) based on the texts of the Qur’an and Sunnah (Prophetic tradition), and scholarly consensus. It was ordained, along with fasting, in the second year after Hijrah (emigration). The recipients of Sadaqat al-Fitr are the same recipients of general Zakaah (obligatory charity), based on what Allah The Almighty Says (which means): {Zakaah expenditures are only for the poor and for the needy…} [Qur’an 9:60]

There is a consensus among the scholars on the obligation of Zakaat al-Fitr, which has two conditions:
1. The amount (one Saa’) should be superfluous over one’s own need and the need of his dependants for the day and night of Eid.
2. It should be given upon (and not before) the arrival of its due time, which is sunset of the last day of Ramadan.
The amount given as Sadaqat al-Fitr is a single Saa‘ for each Muslim, but it can be less than one Saa‘ if the Muslim does not find any more than that. Sadaqat al-Fitr is obligatory upon every Muslim individually, and the Muslim is not obligated to give it on behalf of others according to the majority opinion of scholars. However, one can give it on behalf of those under his care with their consent. An exception to this ruling is the slave, for whom Sadaqat al-Fitr is, of course, an obligation, but in his case it is acceptable for his master to pay on his behalf. The Prophet, sallallaahu ‘alaihi wa sallam, said: “There is no charity to be paid on the slave (i.e. for owning him) except Sadaqat al-Fitr.” [Muslim]
It is recommended that the Muslim gives Sadaqat al-Fitr on behalf of the foetus if it is four months old as ‘Uthmaan, radhiallah ‘anhu, did so, and he was one of the four Rightly-Guided Caliphs regarding whom we are ordered to follow their practices.
According to an authentic narration, it is permissible to give Zakaat al-Fitr early but only by a maximum of two days before Eid as reported from Ibn ‘Umar, radhiallah ‘anhu. In a Hadith (narration), it was said, “Ibn ‘Umar gave Sadaqat al-Fitr for those who would accept it, and they used to receive it one or two days before Eid al-Fitr.” [Al-Bukhari]
However, it is preferable to give it before the Eid Prayer due to the Hadith which was narrated on the authority of Ibn ‘Umar where it was mentioned that the Prophet, sallallaahu ‘alaihi wa sallam, ordered Sadaqat al-Fitr to be distributed before Muslims go out for the Eid Prayer. It is not permissible to delay it until after the Eid Prayer, and if it is, it would not be valid. The Prophet, sallallaahu ‘alaihi wa sallam, said: “Whoever introduces something to our affairs (i.e. religion) which does not belong to it, it will be rejected.”

Therefore, the time for giving Zakaat al-Fitr can be one of three:
1- Permissible: Two days before Eid al-Fitr.
2- Recommended: On the morning of the day of Eid before the Prayer.
3- Prohibited: After the Eid Prayer, when it is not considered valid.
It is permissible to authorise someone to give Sadaqat al-Fitr on one’s behalf. There is no harm in delaying it for a valid reason. Sadaqat al-Fitr can be given out of the common staple food of the country, and according to the majority of scholars, it is not permissible to give the monetary value of Sadaqat al-Fitr. The Muslim may give one person many Saa‘s or one Saa‘ to many people.
It is preferable for the Muslim to give it personally so that he can be sure that it is delivered on time.
The Commander of the Believers, ‘Umar ibn ‘Abdul-Aziz would command the Muslims to give Sadaqat al-Fitr and recite the verse (which means): {He has certainly succeeded who purifies himself. And mentions the Name of his Lord and prays.} [Qur’an 87:14-15]
Ibn ‘Umar said: “The Prophet ordained Zakaat al-Fitr to be given as one Saa‘ (handful) of dates or barley for every slave and free Muslim, male or female, young or old. He ordered it to be given out before the Muslims go to the Eid Prayer.” [Al-Bukhari and Muslim]
Article source: http://www.islamweb.net/emainpage/