With the 5th year anniversary of Kagamine Rin/Len release coming up on the 27th, MIKUBOOK is now asking fans to submit their messages and/or photos congratulating the twins’ birthday. Submissions will be used on a special page that will go live on December 27. Previously, Crypton ran a similar campaign for Miku’s birthday as well.
EDIT: For their birthday, MIKUBOOK now has a special mini-game where you can have Miku provide backing harmony for Rin and Len performing ToraborutaP’s “Omedetou” while the two dance on screen as MMD models rendered using WebGL.