"No Pranks After Noon
This rule appears to be peculiar to English-speaking countries. The custom is that pranks can only be perpetrated until twelve o'clock noon. After that time, anyone who tries to play a prank is himself (or herself) the fool.
There's probably an element of ancient folk belief lurking behind the rule. April Fool's Day honors the spirit of Folly, which is a powerful force. And as such, it needs to be contained within strict temporal limits, lest it overspill its boundaries and cause chaos throughout the rest of the year.
Although there's no known record of this rule having been explicitly articulated outside of English-speaking countries, it's nevertheless widely observed, for a practical reason. People are more likely to be fooled in the morning, when they might not remember what day it is. As the day progresses, they'll wise up, and pranks against them have a higher chance of failing.
The earliest reference to this rule is found in the British journal
Notes and Queries (Aug 11, 1855) by a correspondent who noted that it was a tradition in the county of Hampshire that those who played pranks after twelve o'clock on April 1 would be greeted by the following verse:
April fool's gone past,
You're the biggest fool at last;
When April fool comes again,
You'll be the biggest fool then.
Half-a-century later (Apr. 15, 1905), several more correspondents to
Notes and Queries recalled the rule being strictly in force during their childhood in the mid-nineteenth century. For instance, Harry Hems. wrote: "It is fully half a century ago since I left school, but it is well within my recollection that the practice of playing pranks upon one's fellow pupils on 1 April was not permissible after noontide. Then those who had been tricked by their companions were pointed at by the latter, and the following somewhat dense couplet hurled at them: 'April's gone, and May's come; You're a fool and I'm none!' "
What are the rules and customs of April Fool’s Day?
April Fools' Day