Art experts said on Monday, that there was an astounding haul from Boston's Gardner Museum -- possibly the biggest art theft in US history.
The art theft happened on a Monday. 
It is not said what day the art theft happened. The Art experts commented on a Monday about the theft. 
( z1 thieve :ARG1 ( z2 art ) :time ( z3 date-entity :weekday ( z4 monday ) ) )