when to mail in AOS I-485 if priority date becomes current next month

[KerrBit 0]

If my priority date becomes current next month as indicated on the visa bulletin board, can I mail out the i-485 packet on the last day of this month so it will get to the USCIS lockbox on the first day of next month? Or can I only mail out on the first day of next month? (someone told me the mail post mark matters and if it is post marked on the date the priority date not being current, USCIS will reject it)

[aaron2020 11,651]

No. You apply when the PD is current. One day early means your PD is not current and you are not qualifies yo AOS.

Have patience and wait one day.