When Should You Say `Eid Mubarak?
Question

What is the ruling on offering congratulations for `Eid one or two days before it begins?
Praise be to Allah.Is saying `Eid Mubarak Bi`dah?
Offering congratulations for `Eid is something that is permissible ; that was narrated from some of the Companions (may Allah be pleased with them). 
Ibn Qudamah (may Allah have mercy on him) said: 
“Ibn `Aqil quoted some Hadiths about offering congratulations on `Eid , including the report that Muhammad ibn Ziyad said: I was with Abu Umamah Al-Bahili and some of the other companions of the Prophet (blessings and peace of Allah be upon him); when they went back from `Eid (prayer), they would say to one another: May Allah accept from us and from you. Ahmad said: The chain of narration of the Hadith of Abu Umamah is good.” (Al-Mughni, 2/130) 
When should you say `Eid Mubarak?
What appears to be the case from the actions of the Companions and what has been narrated from them is that offering congratulations for `Eid should come after the `Eid prayer . So if a person limits it to that, that is good, as it is following the example of the Companions of the Prophet (blessings and peace of Allah be upon him). But if he offers such congratulations before that, to be the first to give this greeting to his friend, it seems that there is nothing wrong with that, because offering congratulations for `Eid comes under the heading of customs and traditions, and such matters are broad in scope and should be referred to the customs that are prevalent among the people. 
Ash-Shirwani Ash-Shafi`i (may Allah have mercy on him) said: “From his words it may be understood that on the day of `Eid , it is not required to congratulate people during the days of Tashriq or after the day of `Eid Al-Fitr; rather it has become customary among people to offer congratulations on these days, and there is no reason not to do so, because the aim behind it is to create feelings of friendship and express joy. It may also be understood from his mentioning the day of `Eid  that the time for offering congratulations begins at Fajr, not the night before, even though there are other views, as mentioned in some commentaries. It may be said that there is nothing wrong with doing it if that is the custom, because of what he mentioned about the aims behind it, namely to create feelings of friendship and express joy. This is supported by the fact that it is recommended to say Takbir on the night before `Eid .” (Hawashi Ash-Shirwani `ala Tuhfat Al-Muhtaj, 2/57) 
And Allah knows best.