？
What day of the week are you usually free?
Literal
Always what-day-of-week [if-なら] is-free [question-か]?
A scheduling-coordination question. いつも ('always, usually') opens the question as asking for a general pattern, not a specific occasion. 何曜日 is 'what day of the week' — 曜日 is the counter-suffix for days of the week, and 何 is 'what.' The conditional なら ('if X') is used here to frame the day as a hypothetical condition — 'if it's [day], then you're free?' 空いている ('is free, is open, has availability') uses the resultative ~ている, since 空く is a change-of-state verb meaning 'to become empty/open.' A common conversational opener when trying to coordinate a meeting.