For simple-interest loans, the interest is applied daily to whatever balance is currently outstanding. (If your APR is 5% then you'd have a daily rate of roughly 0.014%.) If you make a payment mid-month then the total interest paid for that month would be reduced slightly because you have reduced the outstanding balance for the last half of the month, but it isn't substantial.
With the every-two-weeks payment plan (which many call "bi-monthly" because most months have ~roughly~ 4 weeks) you actually make 26 half-payments per year - or 13 full payments - since there are 52 weeks in a year. Not only do you get the small reduction in interest due to the more frequent payments, you get a far more substantial reduction because the 13th full payment is effectively all principal.
2016 T250 LWB HR 3.5EB 3.31LS Most all the bells & whistles...
Ordered: 25 Jan 2016
Got VIN: 08 Feb 2016
Window sticker: 25 Mar 2016
ETIS Build Date: 5 Apr 2016
Picked up: 21 Apr 2016