First off there's a Cave-le TV broadcast for Dodonpachi 5:30am April 19 Eastern time hosted by Cave.

Then the Cave Festival itself will be broadcasting from the channels below during the event on Saturday 21st (JST) so don't miss the M2/ketsui talk show from 10:30am (JST)!

https://www.youtube.com/watch?v=V8boM3zhKhs
https://live.nicovideo.jp/watch/lv312373729

And of course, Cave's Steam titles are on sale to celebrate, alongside all the DLC too!
http://store.steampowered.com/app/377860/Mushihimesama/
http://store.steampowered.com/app/314180/Deathsmiles/
http://store.steampowered.com/app/464450/DoDonPachi_Resurrection/