Is there a "best" time of the month to make a principal only payment on a mortgage to maximize the long term savings ?
Before considering adding extra mortgage principal payments, make sure your paying any extra towards higher interest rate loans, like revolving credit cards or your HELOC's. Next, maximize contributions to 401K, IRA's, HSA's, Roth account.
If you've done this, make your regular mortgage paymet PLUS additional principal by the last business day. Remember this posts to your account before the due date. Mortgages calucation the interest based on the outstanding loan balnce over the preceding 30 days.
Example: Regular Payment due April 1, pay by March 31. Your May Statement will calculate interest on the balance carried from April 1 through April 30. The other alternative would be to sign up for biweekly paymets, but these usually have a monthly service fee that you'll need to consider.