Jīntiān shì Yúrén Jié.

今天是愚人节。

It's April Fool's day today.


Notes:

今天(jīntiān): today

是(shì ): is

愚人节(Yúrén Jié): April Fools' Day

Example:

A：今天是愚人节。

     Jīntiān shì Yúrén Jié.

     It's April Fool's day today.

B：愚人节快乐！

     Yúrén Jié kuàilè!

     Happy April Fools' Day.