My guess is this...
Now till Tuesday : Parliament has till next Tuesday to agree an exit deal, any deal, that is acceptable to HMG and the EU.
Wednesday : The EU meet and decide whether to grant an extension based on the deal that Parliament have agreed. If it can be implemented before the new batch of MEP's take their seats in June (because it's a variation of Mays deal), then an extension till then will be granted and we won't have to hold Euro Elections. If it can't be done in time (because it's Canada, Norway, Customs Union etc), then a Long extension (at least 6 months) and we would have to hold Euro Elections.
Thursday : Parliament votes to accept or refuse the EU's decision. If they refuse only two options remain. Unilateral revocation of Art50, or No Deal Brexit. Corbyn tables a vote of no confidence motion.
Friday : Parliament doesn't normally sit on a Friday, but I'm betting it does next week :-) Parliament votes on revoking Art50. And the vote of no confidence.
What I don't know is how much warning the EU need on revocation. Can it be done at 22:59 on Friday? Can Parliament actually order HMG to do something it doesn't want to do without passing a proper bill/law first (as opposed to a meaningful or indicative vote)?