What is the ruling on offering Eid greetings a day or two before its arrival?

Congratulating on Eid is permissible, and it has been narrated from the Companions that they used to say to each other after returning from Eid: "May Allah accept [good deeds] from us and from you." It is preferable to emulate them by offering congratulations after the Eid prayer, but it is permissible to offer congratulations before that, because it falls under the category of customs in which there is flexibility, and its purpose is to show affection and express joy. The time for offering congratulations begins at dawn on the day of Eid.