date -d 'previous Monday" to display the preceding Monday
Use last instead of previous:
$ date -d "last Monday"
Mon May 6 00:00:00 BST 2019
This is documented in info date, and the specific mention of last is in the "29.6 Day of week items" section (thanks to @Jos and @hobbs for finding it):
A number may precede a day of the week item to move forward supplementary weeks. It is best used in expression like ‘third monday’. In this context, ‘last day’ or ‘next day’ is also acceptable; they move one week before or after the day that day by itself would represent.