Here is a list of astronomical events for given month. Location dependent events listed here are visible from central Europe.
May 2015:
4 – Full Moon.
6 – The Eta Aquariids meteors peak with a ZHR of 40.
7 – Mercury reaches greatest evening elongation at 21.2 degrees east of the Sun at 4:00 UT.
14 – New Moon.
20 – The Jupiter moons Io and Ganymede cast a double shadow on planet Jupiter from 22:04 to 22:33 UT.
21 – The Jupiter moons Callisto and Europa cast a double shadow on planet Jupiter from 11:26 to 11:59 UT.
22 – Saturn reaches opposition at ~1:00 UT.
28 – Asteroid (532) Herculina at opposition shining at magnitude +9.
28 – The Jupiter moons Io and Ganymede cast a double shadow on planet Jupiter from 00:01 to 2:18 UT.