In Finland, facepalmsunday refers to the historic parliamentary election day of Palm Sunday 17. April 2011. On facepalmsunday the nationalist populist party True Finns gained 39 seats in the 200-seat parliament, compared to the 5 seats they had gained in the previous elections four years ago. This made True Finns the third largest party, as opposed to the 8th place they held before. It was a huge shock for everyone who did not vote for True Finns.
The term facepalmsunday was probably first used in this meaning in this Twitter message during the vote counting: https://twitter.com/#!/yousifabdullah/status/59702195561570305 The origin as a Twitter hashtag contributes to the fact that the word is written as a compound word, as opposed to two words "facepalm Sunday", because hashtags cannot contain spaces. Also, Palm Sunday is properly written as a compound word "palmusunnuntai" in Finnish.
I almost choked on my drink when the preliminary votes were published on facepalmsunday.