Nowhere was anyone celebrating. "Anniversary" does not necessitate celebration, indeed many in the US commemorate the anniversary of the 9/11 attacks.
True, my point is that a cowardly attack on civilians can only be called a humiliation for the attacker, who resorted to evil underhanded means to “even the score” instead of obeying the rules of war... in the same way the 9/11 and Sri Lanka attackers were filthy cowards.