In Daily Events, you get the same Daily Missions (shown at the Achievements page) as normal, with the addition of getting an event item every time you complete one. The length of the event is shown in the top right (see above) in the format Day: Hour: Minute. The other countdown tells you when you get the next Quest. The Ruby rewards vary depending on the quest level, but the number of event items is always one.
The Event starts at the daily reset at 15:00 UTC (changed from 0:00 UTC on December 10th, 2019). You can use the items to unlock pictures in mail threads, the amount per picture varies on the amount of pics, but completing all missions almost always leaves you with a spare, meaning that you can skip/miss one mission. The extra items currently have no use.