All days have the same length, everywhere on earth, namely, 24 hours. If you're asking about the longest period of time that the sun stays up, that's going to depend on where you are on earth. Here's...
June 21 Updated: Almost every day is 24 hrs, with exception of day light savings when clock is put forward an hour so that day is 23hrs, and when it ends, clock is put back an hour that day is 26 hrs...
Because on that day the sun's rays have the farthest to travel. When the earth begins to shift towards winter the sun's rays are not traveling as far but so the earth gets warmer.