Two things that turn this from a monthly hazard into a non-event.
Ask the sender for ISO format. 2026-07-03, year first. It is unambiguous, every tool on earth parses it the same way, and it sorts correctly as plain text even if nothing converts it at all. It is very often a one-line change in whatever generates the export, and people say yes far more often than you would expect because it costs them nothing. This is the highest-value thing in this thread and it is a five-minute email.
Add a check that fails loudly. Whatever the import method, put a formula somewhere that counts rows outside the range you expect — dates before the start of the month or after the end of it. If the day and month get swapped again, that count goes non-zero immediately and you see it before the report goes out, rather than a month later because a total looked odd.
One more habit worth having: keep the original CSVs somewhere untouched. When you do discover a problem like this, being able to re-import six months correctly is the difference between an afternoon and a reconstruction. If you still have the raw files for the earlier months, you can re-run them properly and find out how bad it was rather than wondering.