Western writers know very little about Islam, so they say that Shia Muslims celebrate Muharram when in fact it is not something to celebrate about. The proper word would be "mourn". As I've said before, I have nothing against Shias (or even Hindus and other no-Muslims), but I do think that the way they observe this month is not quite right. Their chest-beating and self-flagellation, for instance, is incomprehensible, as well as their mourning processions that shut down entire cities.
There was a time, for instance, when all female employees (even Sunnis) of Pakistan TV had to wear black and cover their heads in the first ten days of Muharram, acting as if they'd just embraced Shiaism. Most Shias used to believe that the surest way to get to heaven was to do something that would hurt Sunni Muslims (I know this from personal experience, a Shia government officer deliberately got my salt factory shut down in the sacred month).
Shias of course believe that the ancestors of today's Sunnis killed Hussain, while I used to know many Sunnis who said that it was the ancestors of Shias who did it. I wish they would stop indulging in such fights and get united. I've known many Shias who act like normal human beings even in the first ten days of this month, they don't get upset when a Sunni man says "Assalam Alaikum" to them, but of course there aren't many of them. Most of them are busy mourning.