Mail sent Monday through Friday at a [non-regional processing facility post office] won’t be postmarked until the next day. Mail sent on a Saturday won’t be collected until Monday morning, and it won’t be scanned until Monday afternoon or evening, so the postmark will be two days after it was sent. If a piece is sent on a Saturday before a Monday holiday, it may not be collected, processed and postmarked until Tuesday — a gap of three days.
Please see this link for more details.