As a practical matter, in this particular case you're probably fine. Certificates have expiration dates for a number of reasons, one of which is that there's always some risk of compromise (leaking the private key, somebody factoring the public key, etc.), and regularly rotating them mitigates this. Sites that use manually rotated certificates typically use lifetimes of at least a few months to a year (historically often a few years, but most browsers don't allow that anymore). Let's Encrypt is designed for automatic certificate rotation, so the validity is set relatively short because to the server, rotating every month, or even every day, shouldn't be very hard. In this case, it looks like the server has failed to rotate its cert on schedule, which is a sign of some kind of problem (likely just some sysadmin screwed something up, or some automated maintenance tool got broken), but it's probably not going to put your data at risk.
In general, though, be wary. If the certificate is invalid for any reason other than expiration, that's a very bad sign. If it expired long ago, that's a bad sign. If it's a really high-sensitivity site (e.g. banking, legal, or government site where you're submitting very valuable data), that's a bad sign. If you aren't confident whether or not you can tell the difference between "certificate is valid except it just expired" and "certificate is invalid because it's from a MitM, but also just expired to make me think it was valid yesterday" then follow the recommendation of your browser and don't trust the site.
Of course "don't trust the site" means different things depending on what you're doing with it. Don't enter credentials, or personally identifiable information, or other secrets. (That includes, don't visit the site if you logged in before and the site remembers your identity! That's done by the browser storing and automatically transmitting a secret.) Don't download software. Don't allow any requests for access to stuff like your microphone, camera, or location. But if you're just looking up an owner's manual, or checking a restaurant menu, or checking some public record, it's probably OK. In any case, though, if you aren't sure, it's pretty much always safest to just... not. It might take a few days considering the holidays, but if there's nothing actually wrong, it'll be fixed soon.