There are multiple synonyms to express the sentiment of "may you live to see this happy day". One such phrase is "May you be blessed with the joy of witnessing this auspicious occasion". Another alternative could be "may you live long enough to share in the happiness of this momentous day". "May the stars align and grant you the privilege of experiencing this joyous occasion" is another way to convey the same idea. One could also say "may the universe bestow upon you the gift of witnessing this happy day". Regardless of the exact phrasing, the sentiment remains the same - a well wish for someone to experience a happy and important event in their lives.