OP - Not to ask the obvious, but are you certain that they require a US visa and might not qualify under the VWP for an ESTA?

You have not provided sufficient information for anyone here to provide assistance, including their nationality and travel routing, including all connections.

As a practical question, where are the old passports located and is overnight service available from wherever the passports are located to Canada? Might there be someone who can retrieve the passports and get them to an overnight facility.

"Overnight" might mean Wednesday with the holiday and that might mean changing the flights by a day, but that is better, one hopes, than an outright cancellation.

Also note that visas are typically a consular, not an embassy service. If you are in Ottawa, that will not matter. But, the US has consulates in many other Canadian cities and your parents might be located in or near one of those.