The original event on DMM ran from 4/27 to 5/6 2016. Ultimate mode was added on May 1 and was available for the remaining duration of the event.
http://xn--hckqz0e9cygq471ahu9b.xn--...87%A8%E6%88%A6
http://kamihime-project.wikia.com/wi..._16_-_6_May_16