"Days" before May 16

The untested GLaDOS AI is activated for the first time as one of the planned activities on Aperture's first annual Bring Your Daughter to Work Day. In many ways, the initial test goes well: Immediately, within one picosecond of being switched on, GLaDOS becomes self-aware. The "going well" phase lasts for two more picoseconds, at which point GLaDOS takes control of the facility and locks it down, trapping everyone inside. She then deploys a deadly neurotoxin which kills the majority of the scientists within the facility. A few survivors manage to install a Morality Core into GLaDOS which prohibits her from using the neurotoxin again. With the Morality Core in place, GLaDOS turns her attention back to the facility and begins testing. Her goal: beat the hated Black Mesa in the race to develop a functioning portal technology.

From the Timeline of the Half-Life and Portal universe article on Combine OverWiki.