Can I still catch darkrai at this Halloween event ?

Thats alittle vague, Darkrai is currently catchable in this event and if you've previously caught darkrai you will get the pumpkin form version instead need to know anything else let me know :)

So now what I have to do to catch it I’ve done the p1 of event on 2017 but I didn’t catch it . From where to start ? Should I do the p2 of event ?

If you've done part 1 of the quest then you start in dreadtown to the right of the mayor in the big building will be a hole in the wall go through that and you should be able to begin the second part of the quest when you reach necropolis you will need a poke with miracle eye and either odor eleuth/foresight GL with your quest :)

If I finish the 2nd part of the event I can catch darkrai?

Yes you can

Thank you very much mate gl too