X-day was Umbrella's term for the S.T.A.R.S. mission to the Arklay Mountains. Albert Wesker and Sergei Vladimir were tasked with removing evidence of Umbrella's experiments before S.T.A.R.S.' arrival.
Wesker's orders were changed on July 24, when it became known that the S.T.A.R.S. Bravo Team had survived their chopper's crash (Wesker had sabotaged it). Hoping to avoid any searches for the missing members being conducted, Wesker led Alpha Team to the region himself.
Upon arrival, Wesker began searching for and destroying incriminating evidence. When one of the surviving Bravos (Enrico Marini) became wise to a traitorous presence among S.T.A.R.S., Wesker murdered him.
Upon his arrival at the Arklay Laboratory, Wesker was tasked with collecting at least two samples of each of the B.O.W.s besides the Tyrant (T-002 Type), which he was to dispose of. He had anticipated this as an opportunity to leave Umbrella by way of faking his death, using a unique virus (possibly a G Virus variant) engineered by William Birkin.
Wesker waited for the survivors to discover him, at which point he injected himself with the virus and proceeded to awaken the T-002. The creature impaled Wesker with its claws before hounding the survivors.
The scheme worked, and the virus saved Wesker. Upon awakening, he activated the self-destruct system and escaped the area (accosted and pursued by Lisa Trevor along the way) through the mansion's front door.
Umbrella branded X Day a failure due to the S.T.A.R.S.' survival, Wesker's supposed death, and the lack of surviving samples.
Evidence exists that suggests that any person with involved in the Mansion Incident was to be killed or silenced. Several attempts were also made on the lives of potential courtroom witnesses.
- Bravo team's helicopter was sabotaged to crash.
- A page of a Will that described Wesker's appearance at the mansion was torn out.
- Enrico Marini was assassinated to prevent him confirming the traitor's identity.
- People's faces on portraits on the Eastern staircase were painted out.
- Barry Burton was blackmailed into killing other S.T.A.R.S. members.