Hi, I saw your question in the shout box, to answer in case you don't have one, there is no specific date for Ola Ola, it's totally random. But you can check in-game when will be the next event, doing @event. It will gives you the time of the next event. By then a notice will appear at the top of your screen saying "the <name of the event> will be in 4 hours" or something like that. Hope it helped!
o/ there is a specific time for Ola Ola (however i am not very sure for Ola's, i know piggy piggy will start at 1900 SGT). it is rotated with other event. at the same period you logged in you will always find the same event. it's right every event will start 4 hours intervals! you can follow marie's teaching to @event!