at the occasion意思为：在......之时。
例句：
The full moon rises at sundown today.
今天满月在太阳落下之时升起。
They were to share that with everyone at this time.
他们将在会合之时分享这些。
occasion意思为：
一、在......场合。
例句：
Taking her with me on official occasions has been a challenge...
带着她出席正式场合对我来说一直是个挑战。
It will be a unique family occasion.
这将是一次特别的家庭聚会。
二、机会；时机。
例句：
It is an occasion for all the family to celebrate...
这是个值得举家庆祝的时刻。
It is always an important occasion for setting out government policy.
这历来都是阐明政府政策的重要时机。