Yes, Netherlands and Great Britain will be the first countries to vote. Some others on Friday and Saturday, while more than a half countries, among which Italy, will vote on Sunday.
I've read on Ansa that the referendum will deal with divorce, so I think this will probably cause a sharp rise in turnout, even though in Ireland it is already relatively high. Here in Italy there are also local elections and almost 75% of them will be on the same day of European elections, but other will be split all over the years and some had already occured, (with bad results, from my political point of view, but this is another story)
It looks like you have other things in common other Totallympics :D however, I see that you vote earlier as volunteer in polling stations. Here in Italy, instead, they vote in the same days of regular electors.
Have a good vote :D
Nothing to say, I just didn't want to be unfair :D