4th July on Saturday when is USPS closed?

I would suspect that, for many people, even if your mail is picked up on the 3rd, unless your carrier scans it, it might not get scanned until Monday, so plan accordingly. (of course, for us, it’s pretty much the norm that packages do not get scanned soon enough for Amazon; to Amazon, a scan at 12:20 am after being picked up on the Ship Date is a late shipment).