It's usually around the 5th - I never bother to check myself though. I'd guess Monday, but if you know your ART ID, you can log in on the student finance direct website and I think that has the payment schedule.
With SLC loans it's always on the first date of 'term' even though we don't even have terms any more. So first Monday of Week 1, first day of Revision Week and first day back after Spring Break.
My schedule says the 5th, and when it's been a weekend in the past, it's always gone in the day before, but there's no sign yet for me. I'm guessing it'll be in on the 7th then.