It isn't really a hard puzzle. Just a stupid one. We have no way to know that e5 was blacks last move. So we have no idea that e.p is possible. Puzzles like this only works if you can deduce that e.p is allowed.
If this position was the puzzle of the day...The puzzle would be easy to solve , as blacks"s last move would be indicated. Only a uneducated beginner would fail to find the mate, ignorant of the enpassant rule. It takes some deductive reasoning to solve this puzzle with out a clue. Maybe Sherlock Holmes composed this problem... I failed to find the assumption needed to solve it, and its funny that some GM'S have too! Nice post #1 :]
In puzzles, en passant is assumed to be illegal unless you can prove that the pawn moved two squares on the previous moves.
