Ruling on offering congratulations for Eid before it begins

Offering congratulations for Eid is something that is permissible; that
was narrated from some of the Sahaabah (may Allah be pleased with them). 

Ibn
Qudaamah (may Allah have mercy on him) said: Ibn ‘Aqeel quoted some hadeeths
about offering congratulations on Eid, including the report that Muhammad
ibn Ziyaad said: I was with Abu Umaamah al-Baahili 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 isnaad of the hadeeth of Abu
Umaamah is a jayyid isnaad.

End
quote from al-Mughni (2/130) 

What
appears to be the case from the actions of the Sahaabah 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-Shirwaani
ash-Shaafa‘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 Tashreeq 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 takbeer on the
night before Eid.

End
quote from Hawaashi ash-Shirwaani ‘ala Tuhfat al-Muhtaaj (2/57) 

And
Allah knows best.