The entire event is from the 9th to the 22nd. You can start any time you want during that timeframe. However once you start it is a hard 10 days only to accomplish. On the 12th the event will automatically start for everyone who hasn't voluntarily started already. Some people delay the start so they can time it with days off from work, weekends, upcoming personal issues in RL etc. if you voluntary started on the 9th then the event will end 10 days later on the 19th for you. If you start on the 12th then the event will end 10 days later on the 22nd. However if you start on the 13th or later, the event will still end on the 22nd but you will have less days to finish everything.
Each day will start at the same time. If you accomplish your daily goal, the event will freeze until the reset for the next day. If you do not finish the daily goal then your day will not freeze and will roll over into the next day. Rinse repeat. If you fall behind a few days, it will become harder and harder and most unlikely to finish the entire event without using gems. Therefore I would encourage everyone to finish out each day on that day.
Hope that helps