Veselé Velikonoce roku 2008. You can read a bit more about Czech Easter in this post from last year or in one of the two comprehensive articles below.
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Easter Monday practices, involving boys pouring water over girls and lightly whipping them with braided branches, are the most controversial of Easter traditions. Global Voices Online » Czech Republic, Slovakia: Whipping Girls and Other Easter Traditions
Velikonoce probably comes from velika noc or Veliké noci (the former being Great Night, the latter Great Nights) symbolizing the greatest night of the year, the night of the Resurrection of the Lord. I wouldn't go with Happy Midnight because happy is Vesele/st'astne and midnight is půlnoc.
VESELE VELIKONOCE! Kristus vztal z mrtvých!
Veliko = Great
Noc = Night
My understanding of the phrase is that without the resurrection, christianity would not exist, so the night of the resurrection is considered a great night, and the resurrection a great feat.
Could be wrong though. These kind of names have been handed down through the centuries and it's difficult to pinpoint their origins the way we can today, such as the first use of the term 'Iron Curtain' for example.
Hope this helps...