2500 years ago, Oshaka-sama was given a birth in Lumbini, present Nepal. Right after he was born, he walked severn steps in every quarter and said "「天上天下唯我独尊」(Tenjo tenge yuiga dokuson)", which is a very famous story now. There might be no need to say that a fact that Oshaka-sama, founder of Buddhism, attained the enlightment always bases our belief in Buddhism.
This is a day that we appreciate the birth of Oshaka-sama. Also it's
when we can express our appreciation for being able to join in this prayer.