Where did April Fools Day get it's name?

It's April Fools Day, I've oftened wondered where that title came from so I tried looking it up online. Now I'm more confused than ever! This is what Yahoo had to say:

Dear Yahoo!: How did April Fools' Day originate? Asril
Dear Asril: Apparently, you aren't the first to ponder the origins of April Fools'. While researching your question, we stumbled across this poem:

The first of April, some do say,
Is set apart for All Fools' Day.
But why the people call it so,
Nor I, nor they themselves do know.
But on this day are people sent
On purpose for pure merriment. --Poor Robin's Almanac (1790)

It's an apt sign of the ambiguity surrounding the holiday. As we discovered in our search, everyone has a theory, but few agree on how the tradition arose. Still, we'll try to get as close to the truth as possible.
We began in Yahoo!'s April Fools' Day History category, under Holidays and Observances > April Fools' Day.
The first site, Snopes, has an interesting theory. Many believe that up until the mid-sixteenth century, France celebrated the new year on April 1. But in 1562, Pope Gregory XIII introduced a new calendar to the Christian world, which changed the date of the new year to January 1.
Of course, they didn't have email back then, so word traveled slowly and some people continued to celebrate on April 1. These poor, misinformed souls were referred to as "April Fools" and had tricks played on them.
The second site, Holiday Page: April Fool's Day, offered a similar explanation, although the details differed a little. According to their version, Charles IX introduced the Gregorian calendar in 1564, changing the date of the new year. The "April Fools" tradition was then passed on to England and Scotland in the eighteenth century and eventually made its way to the American colonies.
Next, we checked out a message board devoted to the origin of April Fools' Day, but it just added fuel to the fire. In the end, further research only led us to futher debate. We concluded that there is no absolutely conclusive conclusion. Whatever the specifics, we have to agree with Mark Twain, who said, "The first of April is the day we remember what we are the other 364 days of the year." 

Ok what do you think? Is it as clear as mud now?....I thought so.