Ms. Day was a communist, and no saint. Her main concern was social justice causes, for some time the progressive left has claimed that wealth redistribution is in fact an extension of Catholic doctrine. You may read the myriad of articles written by leftist Catholics starting at the Daily Kos concerning this. It appears that this is the case given the elevation of Ms. Day to canonization by every leftist Catholic.
Mary is reviled by leftists who nominate one of their own as a saint, and the Catholic hierarchy plays politics and approves it.