This was right around May 15th, 1948. On this date the British mandate was due to expire. Since May 15th 1948 was on a Saturday, the temporary committee members including Ben Gurion decided to move the declaration up to Friday afternoon. So it was, that on May 14th, 1948, a gathering was assembled in this museum. On this day Ben Gurion declared the establishment of the new state, the State of Israel.
During a visit to this museum you can watch a film that describes the events of those days, on the establishment of Tel Aviv and of course the declaration of the State of Israel. You can see here the museum auditorium that was maintain more or less the same from that day, and can imagine Ben Gurion declaring the state. Religious enthusiast are welcome to go to the top floor and see religiously inspired art pieces.
Then Dizengoff moved to the small apartment that was built on the building's roof. In his will he donated the building to the children and residents of Tel Aviv. Since 1936 the house operated as an art museum in Tel Aviv. In 1948 it received its biggest promotion, and became a historical site. It's not every building that a declaration for a country happened inside of it.