[a pedant writes]
*Plenary session of the European Parliament.
You're right that it's that actual deadline, but it's also important to remember what has to happen to get to that stage.
The reason that the Council meeting just gone was important is that it was the last opportunity for Barnier's mandate to be changed. And it didn't, so there is no room for the EU side to move very far now.
But for the Parliament to ratify any potential agreement, a draft treaty needs to emerge, go through translation, go through committee stage etc. And none of that is an overnight thing.
So for a deal to happen, it needs to UK to (broadly) agree to the thing on the table at the moment. I'm still 51-49 on the side that this is UK theatre before capitulating like Johnson did last Autumn, but there is a long of embarrassing rowing-back that will have to happen from the UK side to get there. And it'll need to happen pretty quickly to get through the necessary parliamentary stages.