The double door will be opened from 8PM on June 19 till 4AM on June 20, JST, according to TEPCO's press release on June 19 (in Japanese).
TEPCO has been running the air filtering system in the Reactor 2 reactor building in order to lower the radioactive materials in the air inside the building since June 11. The company is hoping to reduce the amount of radiation to one-tenths before they open the double door to the reactor building.
The Reactor 2 reactor building shell is relatively intact, causing the very high humidity and temperature, hindering any work by carbon-based workers inside. By opening the double door, they hope to drive out the moisture and humidity enough so that they can send in the workers.
Well, the amount of radiation doesn't seem to have dropped at all, as you can see in the chart below (by TEPCO, as a handout at the press conference on June 19), but they are going to open the double door anyway. It was the same story for the Reactor 1; the air filtering system didn't lower the radioactive materials in the air, but they opened the double door anyway, claiming the level got lower.
All for the show, except it exposed the workers to unnecessary radiation when they were sent in to install the air filtering system.
If TEPCO knows it is a futile effort to install air filtering system before opening the double door, why does the company do it? It's for the public consumption, to show that TEPCO is doing everything they can to ensure the lower radiation level for the workers and for the community (which has all but evaporated). And to keep the regulatory agency, NISA, happy.
And why do they open the double door at night? As I said when they opened the double door to the Reactor 1 reactor building, this one is for the maximum effect of letting the inside "hot" air out as much as possible, using the air temperature difference between inside and outside. The relatively intact building shell may hinder the air exchange though. We'll see.
I'll look forward to seeing the gamma-ray photos of the reactor.