For around 4 weeks either side of June 21st in the Northern Hemisphere tat latitudes between 50° and 70° here is a chance to spot a naked eye phenomenon termed "Noctilucent Clouds", which literally means night shining clouds. NLC's (for short) are clouds form so high up in the polar mesospheric clouds in the upper atmosphere that it is thought they exist as water-covered meteor dust. Well after evening twillight looking at where the sun has set (or going to rise) with no clouds and you might spot the silvery-blue display - you'll know if you see them!
You can read more from any of the previous threads showcasing previous displays here.
Photos of one of the great displays on June 17th 2014 by Seanie Morris.