Supercharge.info lists the opening date as 2014-08-08.
I'm not sure what this information is based on, as the TFF-Forum does not have any post on that exact date.
All building seems to have happening during 2014. There are pictures of the construction July 12, July 16, July 19, July 24, July 31.
On August 6, 2014 user an75re shared pictures of the mostly completed Supercharger, with the EVite charger still being installed. At that time only the Evite parking spaces are painted green. The Supercharger spaces just have a yellow square and the Tesla vertical sign.
On August 6, 2014 as well, user elekktrisch shares pictures of their car plugged in but not receiving any power.
On August 10, user stealth_mode is the first to indicate they charged successfully. There's also a picture of the completed EVite charger.
On August 15, user Steph announced that the official opening was that day, along with a picture of the ribbon cutting.
On October 22, user Sierra shared a picture of the Tesla screen showing the charger was closed on October 22 and 23, no idea why.
There aren't any good pictures we see the entire setup at once, but by piecing together what we have we see there were 6 stalls originally. The EVite charger was on the 7th and 8th spots immediately to the right of the Supercharger, which means it was moved when the Supercharger was later upgraded.
On April 2nd, 2017, user P851 shared a picture of multiple crates deposited next to the Supercharger. On the stalls there is a sticker reading "Achtung! Anlage am Dienstag 04. April ausser Betrieb".
On April 19, user Stormtrooper_MX90 shows pictures of the construction work. At this time it seems like the EVite station has already been moved 4 spots to the right and a car appears to be plugged in, so it must be connected to the grid.
On April 24 user TeslaFanCH shared pictures of the partially completed upgrade. The 6 original spots seem to be working as a car is plugged in. The 4 new chargers are covered by a plastic sheet. The new parking spaces for Tesla and EVite are not yet painted.
I can't find any comment or picture confirming when the new stalls were put into service. It must have been around April or May 2017.