Austrian broadcaster ORF is aiming to host the Eurovision Song Contest 2015 on the 19th, 21st and 23rd of May next year. They have published these days as their prefered date in their official document that states the requirements to the venue of the contest.
The first preliminary dates for the contest were one week earlier, on the 12th, 14th and 16th of May. These dates are still an option, because they are now marked as temporary alternative dates. ORF also holds open the possibility for the dates 5th, 7th and 9th and the 26th, 28th and 30th in May.
You can read about all the other details that the venue for the Eurovision Song Contest should require here.