The legendary Latios will be available in Raid Battles in a Special Raid Week that lasts from Monday, April 15, 2019 through Monday, April 22, 2019. Additionally, there is a chance that Latios may be Shiny while attempting to catch it.
Specifically, it will start appearing at 1pm PDT (GMT-7) on April 15 and will then be phased out on 1pm PDT (GMT-7) on April 22. You’ll only have that time frame to challenge it and catch it before it flies off until it makes its return someday in the future.