You suggested an available date and people are telling you they might not be able to make it. Did you want everyone to pretend they could make it until it was definite, then tell you they couldn't make it?
This poll should probably be 'when isn't good for you?' because now you're just getting into a process of narrowing a date down the long way round. OMG DavPaz! Why do you even bother!!?!?!
I can do any of those and flights for March are currently about £50. By January they'll be over £100, I'd expect. So whatevs.
18th/19th is the weekend before my Birthday (so, it's a Monday next year) so probably not for me that one, but yeah, I think you need to allow multiple votes, so that people can select ALL the days they can make it.