Alegrias1
So when Johnson tells us he didn't know anything about the party, he's either lying (which gets my vote) or he was repeatedly hoodwinked by a group of people who worked for him. And he was suitably convinced by people lying to him that he felt OK coming on national TV to tell us there was no party, that it was all legal, that all the rules were followed.
That's certainly not the kind of person I want running the country.
Lying to parliament isn't the only reason for a PM to resign. Being gullible to anybody who tells him what he want to hear, or else being a mendacious liability who despises the people he is meant to be working, they are pretty good reasons too. And one of those definitely applies.
He may not have attended this particular party, but that is not the only party held in no 10 including the flat.
He is quite content for focus to be entirely on the 18th, as the alternative is even worse.