Thankfully, my most senior staff person answered the patron perfectly. She explained that if we close on every Monday holiday, the part-time staff loose too many hours. When we are open, they get more pay than when we close. The patron nodded and said, "Oh, that's a good thing, then."
I say this sincerely, God bless the staff person who took what could have been perceived as lack of respect for the African-American civil rights struggle and turned the interaction into a chance to show how the library helps people with lower incomes.