Yeah, you told me about it.

Day One was the OEM version of Half-Life that was bundled along with some hardware and was also available as a package called Half-Life: Initial Encounter, that bundled Day One and Team Fortress Classic.
Usually, all the games play out parallelly, with the exception of Uplink, which is kinda set in an alternate universe or something and has very little to do with the actual game. (In fact, next to nothing) But otherwise chronologically, it goes this way:
Half-Life: Blueshift begins
Half-Life begins
Half-Life: Opposing Force begins
Half-Life: Blueshift ends
Half-Life ends
Half-Life: Opposing Force ends
Talk about complicated.