GoCardless's day_of_month subscription parameter only accepts 1–28 — full stop, no month-aware rounding on their end. The plugin's UI enforces the same 1–28 range for that reason; there's no way to ask for "always the 31st" or "last day of the month" through this field. If you need end-of-month billing, you'd need to build that logic yourself against the GoCardless API directly (their -1 sentinel for "last day," where supported) — it isn't exposed here.