No Fred, in the US April 1st is the day of the year where you play a little joke or prank on people and it is funny if they fall for it because every body knows April 1st is "April Fools Day". So Everybody wants to be on the lookout for people tricking them on April 1st, but many people forget it is April 1st. Even if they don't forget sometimes they still get fooled. I always remember it is April 1st, because it is an anniversary for me but even I fell for it yesterday.
My friend who recorded something I wanted to watch very much on TEVO for me back in October, kept forgetting to burn it to a DVD for me and I have been remdining her but she never remembers. So finally she called and said she finally did it and would give it to me when she came over. So she comes over and she says 'I can't believe it - I forgot the DVD". I said "WHAT"!!!! and of course, she said APRIL FOOLS DAY!!! HA HA HA. I thought damnit! She got me.
If I misunderstood your post and you already know what Aprils Fools Day is then sorry about that. Knowing you are in Sweden, I thought maybe you might not know the full meaning of the day and I didn't want you to think it was only for atheists so please don't be offended if I am explaining something you already know.
As far as people implying that they have changed their world views on Facebook on April Fools day, I think that is pretty funny. Believe me, many Christians would like it even if they knew it was a joke because it would be a statement that gives glory to God even if the person was being dishonest.