To celebrate this day, recommend a book to everyone here :)
A Brief History of Time by the wheelchair guy.
For anyone who can track down a copy, I'm going to recommend Steven Runciman's account of the 1453 siege of Constantinople. It's not the best work historically, but it's just absolutely gorgeously written and it's the point at which I think the Byzantines really caught my imagination (and thus indirectly responsible for SO MUCH of what's around you now!)
I think my Byzantine bug was set off by Guy Gavriel Kay's "Sarantine Mosaic", comprising two novels Sailing to Sarantium and Lord of Emperors. They're fantasy, set in a slightly disguised Byzantine Empire at the time of Emperor Justinian (about 535AD). The ending is very different from the real history, though.
I got more historical background from Tom Holland's In the Shadow of the Sword - it's a non-academic history looking at the area of the Byzantine and Persian Empires between roughly 400 and 750AD, and the development of Judaism, Christianity and Islam in that period.
I recommend all these books as well worth a read.
I've been meaning to read Kay's shizzle for a while, may well check out the actual history things as well.
Don't know much about Byzantine or Slavic themed books so I can't think of anything to recommend specially for today.