Truth is we really don't know where the tradition of playing pranks on people came from.

There are a few theories floating around though.

When the western world used the Julian calendar, the year began on March 25th because they celebrated the start of a new year with the start of spring. However since that fell in the Holy Week, the celebrated it on the first of April. However when we switched to the Gregorian calendar in the 1500s, we moved the New Year to the first of January. According to the most widely told story, those who still celebrated the New Year on April 1st were called April fools.

The Encyclopedia of Religion and the Encyclopedia Britannica thinks that the timing of April Fool's Day is directly related to the arrival of Spring, when nature 'fools' humans with erratic weather.

The Country Diary of Garden Lore has a theory that April Fools Day commemorates "the fruitless mission of the rook (the European crow) who was sent out in search of land from Noah's flo ...more

Source: google search

The Origins Of April Fool's Day - April 1

What is April Fools Day and how did it begin?
Well, that is a very good question. The origin of this holiday is rather uncertain.
However, the common belief holds that during the reformation of the calendar
the date for the New Year was moved from April 1st to January 1st.
During that time in history there was no television or radio so word spread slowly.
There were also those who chose to simply ignore the change and those who
merely forgot.
These people were considered "fools" and invitations to non-existent parties and ...more