Most employers use the scheme, 15th & 31st.
I've never heard of any using the 1st or 31st, and the 15th.
Are you sure that is the scheme used by your employer?
Yes, if a payday is the 31st (as it is this month), the payday is usually the business day prior to the weekend or holiday.
So, the 31st becomes the 29th.
You should be able to deposit the check today, the date, notwithstanding.
If you tried to cash the check, that could be a problem, and a potential violation.
However, your employer may interpret the payday weekend rule, as being the day after the scheduled day.
So, in your case, the 31st, becomes the 1st.
As long as that is published policy, I don't see a legal issue.
Why not ask the agency in WA that enforces payday laws?