Just looking at it as a layman, it seems Summer or holidays can work equally well. "Avatar" was a December release, and it's quite likely the sequels will come out in December 2014 and 2015. The recent Bond movies were November releases. "Titanic" was a December release. Four of the Harry Potter movies were November releases. TMP, TVH, TUC, and FC were November or December releases and are four of the best-received and top-grossing Trek movies.
Of course, if "Avatar 3" is coming out in December 2015, then it probably isn't a good idea to release the cornerstone of the fiftieth anniversary celebration of Trek in its shadow. December 2016 would be too late. Given the anniversary, the Goldilocks choice is Summer 2016.