This should begin with either On your sixteenth birthday or At your sixteenth birthday party. On your birthday party is wrong. (Many people say prepositions are the most difficult part of the English language; they’re probably right.)
Also… an ellipsis has three dots, not two. It’s bad enough to end a sentence with an ellipsis when the sentence doesn’t trail off, but if you must do it, do it with real punctuation.
Of course, you already know that a word such as sixteenth should be spelled out, not written with a numeral.
Because the birthday party itself seems relevant to the situation, I’m going with the second option for the beginning of this example:
At your sixteenth birthday party, you hear your mother’s voice in your head. Looking across the room, you see her smile as she silently explains that all women share a psychic connection when they come of age. This secret silent language has been hidden from men for millennia.
(Despite what Mercenary Proofreader’s friend Blue used to say, there is no evidence that psionic — or psychic, if you insist — abilities are gender-linked.)