一期一会 which means one of expression of feeling like you should take care of every moment when you meet everyone. It may not come again.

「一期一会」： "for this time only," "never again," or "one chance in a lifetime."

"for this time only," "never again," or "the one chance in a lifetime."

就是通过一系列的茶道活动，包括水、饭、谈、茶四大步，最后完成时使亭主和主客、从客静心清志，由内到外自然涌现出一种“一期一会、难得一面、世当珍惜”之感，苍凉而略带寂寥。

It is not really a proverb but a 4-character idiomatic expression (四字熟語 or yojijukugo). It is read as ichigo, ichie. The closest English expression I can think of is "once in a lifetime." I bet you know what that means.