"二十五周年纪念日" can be translated into English as "the 25th anniversary" or, more formally, "the 25th-anniversary celebration/day" (if referring to the specific day or event of the celebration).
For example:
We are celebrating our 25th anniversary this year.
（我们今年在庆祝二十五周年纪念日。）
Today marks the 25th-anniversary day of our company's founding.
（今天是我们公司成立二十五周年的纪念日。）