..Or, How to say Hello in Elvish (Tolkien Sindarin to be exact).
Mae Govannen translates to "Well Met" and is general purpose greeting for those of Middle Earth who speak Sindarin. If you want something a little more cheery to say, "Mae Govannen, mellon!" is "Well met, friend!". "Mellon" shoud be recognised by any Tolkien fan as the word for friend, as it is the answer to the riddle Speak friend and enter, presented by the door of the Mines of Moria that so perplexed Gandalf in The Lord Of The Rings. You can also use "mellyn" for "friends", "meldir" for friend, male and "meldis" for a female friend.

If you're in need of something a little more intimate, "Mae Govannen, melethril/melethron" (female/male) would be "Well met, lover", and "meltha/melon" is "beloved"...although "well met" doesn't seem like the most appropriate thing to say to your SO. Those elves can be a bit formal though, so you never know.

And when it's time to say goodbye, "Maer Aur" or "Maer Fui" shoud serve you well as "Good day" and "Good night". Now, hopefully, you won't be so at a loss next time you're at a fantasy convention and want something smart to impress people with.