Answer:
Basically, the museum might price this way because of the low number of people that go to the place that specific day and in order to stimulate the demand from visitors.
Step-by-step explanation:
To begin with, the reason why organizations price this way, in where they offer one day total free admission is due to the fact that commonly that day selected does not count with much visitors in comparison with the other days of the week supported by statitics of the place and therefore the museum, in this case, wants to increase the amount of people that enter the building, basically.
Secondly, once that the main reason of why the museum might price this way is said, the reason why the are likely to choose a day like Thursday rather than Saturday is due to the fact that Saturdays tend to be the day where those kind of places are full of people because most of the people do not work on weekends and therefore the organization do not need to encourage the entrance of visitors, they will go anyways without any stimulation.