Isn't Monday too late? Doesn't the agreement with the EU stipulate that there needs to have been some real progress by today to justify extending until May 22, and if no real progress occurs by today, no deal Brexit occurs April 12?
If the deal isn’t passed today, the April 12th deadline is for the UK to come up with a new suggestion.
If the deal fails, the UK could request a longer extension, or even revoke article 50.
The EU has strongly signaled that they’d be open to adjusting the political declaration in favour of a customs union, for example. If the UK were to vote for that early next week, it could in principle be agreed on both sides fairly quickly.
The main EU requirement is that to avoid a no-deal exit on April 12, the UK has to actually do something different, not simply ask for more time. If the UK requested time for a general election or a second referendum, that would probably be accepted by the EU (although it’s not guaranteed).
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Edit: See comments below - I got the dates wrong.
With a deal, the deadline is May, allowing time for some of the necessary domestic legislation.
Deadlines April 12 -- if mays deal has passed, extended to May 22 to allow domestic legislation to be passed to implement the deal
If not passed by then, then uk leaves with no deal
However May's government can ask for an extension at any point up until April 12th. This extension will rely on the UK participating in the EU elections. Those elections can be held if announced by or on April 12th -- there's a reason it's that day and not April 3rd or 19th.
It's almost certain we'll be looking at a long delay, and either a referendum or general election