Depending on where you live, your city/country/region may celebrate Eid as one day (North America in general), or three days (India/Pakistan and much of the Arab world).
As far as I know, there is no specific legislated number of days that 'Eid should be in shari'ah. However, I put that question out there: is there any legislation on how many days Eid should be? (Either or both Eids).
I know that there's a hadith that talks about public holidays in Islam, and it mentions that we only have "the two Eids." But it doesn't mention a number of days.