There is no day you can enter for free.
Musuems in London are free except certain paid exhibitions they may have on.
I visited on a Saturday in mid march 2018 that was free. Keep an eye on their calendar. Not very often, but there are a few free admission days.
Yes there is. I visited on a. Wednesday in the spring, so i am guesssing every wednesday in the spring at least
Everyday need to pay to enter, no free entry
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