Is Dorothy Day considered saintly because of her support for Fidel Castro?

Of course not. Holy people are admired for their sanctity, not their politics. Dorothy Day (1897-1980) was a journalist in the early part of the 20th century who converted to Catholicism. She had lived a sinful life for a number of years prior to her conversion, including having been involved in two common-law marriages and having had an abortion. After her conversion she became involved in social justice campaigns and started the Catholic Worker movement. She did indeed take stances on political issues that would today be considered leftist, but her main concern was to care for and be an advocate for society’s poor and marginalized. When the 1960’s “free love” revolution came, Day rejected it by saying that she had already lived through the bad effects of “free love.” She also remained staunchly loyal to the Church. The Archdiocese of New York opened a cause for sainthood for Day in 2000. She is currently given the title Servant of God.