There is never really a good time for their site to go down, but they have to do maintenance sometimes.
That said, I don't remember seeing anything announcing this ahead of time. It is possible this was unplanned? Maybe related to the recent AWS outages?
Assuming it does come back up on 12/22 as scheduled, that still leaves you a full week (even accounting for holidays) to get your filings submitted. If this downtime really does prevent you from filing on time, you could try asking them for an extension, or at least a waiver of any penalties. They are usually pretty reasonable.