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.