The time when zakaat al-fitr becomes obligatory begins at sunset on the last day of Ramadan. The Sunnah indicates that it is permissible to give it one or two days before the day of Eid. See the answer to question no. 37636
The Hanafis are of the view that an analogy may be drawn between zakaah of one’s wealth and zakaat al-fitr, so they say that it is permissible to bring it forward.
As-Sarkhasi said in al-Mabsoot, 3/110: The correct view in our opinion is that bringing it (zakaat al-fitr) forward by one or two years is permissible. End quote.
However, what appears to be the case is that bringing it forward ahead of the time prescribed in sharee‘ah is not permissible.
There are two significant differences between zakaat al-fitr and zakaah on one’s wealth:
The reason for zakaat al-fitr becoming obligatory is sunset on the last day of Ramadan, whereas the reason for zakaah on one’s wealth becoming obligatory is possession of the nisaab (minimum threshold of wealth at which zakaah becomes obligatory). Bringing something forward and doing it before the reason for it comes into effect is not permissible. Hence if a person hastens to pay zakaah on his wealth before he has possession of the nisaab, that is not acceptable.
Ash-Shiraazi said in al-Muhadhdhab: It is not valid to pay it in advance before the month of Ramadan, because that is bringing it forward and doing it before the two reasons for it [i.e., fasting and sunset on the last day of Ramadan]. That is like paying zakaah on one’s wealth before one has acquired the nisaab and one full (hijri) year has passed since that date. End quote.
See: Manzoomat Usool al-Fiqh wa Qawaa‘idihi by Ibn ‘Uthaymeen, p. 143
The purpose behind zakaat al-fitr is make the poor independent of means on the day of Eid, but this purpose is not met if zakaat al-fitr is brought forward. That is unlike zakaah on one’s wealth, the purpose of which is to make the poor independent of means in general terms and is not limited to a specific time.
There is also a third way in which we may differentiate between them, which is: zakaat al-fitr is a small amount, so it does not serve a great purpose for the poor person if it is brought forward, unlike zakaah on one’s wealth.
And Allah knows best.