First of all, Hussein (radhiAllahu anhu) is the leaders of the youth in Paradise, and we praise him since he is the beloved of the Messenger of Allah. Anyone who claims to follow the sunnah and says evil about the Messenger of Allah's grandson is a person who is deviating.
Now to the issue of ashoora. This is the day when the grandson of the Messenger of Allah - Hussein ibn Abi Talib (radhiAllahu anhu) was killed, and became a shaheed.
But why is it wrong in comemmorate this day?
First, there is no evidence or practise from the Qur'an or the Sunnah of the Messenger of Allah or any of his companions that this day should be comemmorated - like the shi'a do - of beating yourself up etc.
The religion of Islam is based on two principles: that we should worship nothing besides Allaah Alone, and that we should worship Him in the manner that He has prescribed, not by means of bid’ah or reprehensible innovations. - Similarly, adding something extra to the religion by harming ourself on this day will be forbidden, since the Messenger of Allah did not perform this practise. What the Messenger of Allah and his companions did in matters of religion we should follow, what they did not do are not part of the religion.
They (the shi'a) don't comemmorate the death of other pious heroes of Islam, which they themself accept - like the death of Hamza, Khadija, or even the Prophet himself!
The shi'a don't even comemmorate the death of Ali, who is greater in the eyes of the shi'a than hussein.
Why do they comemmorate the death of hussein and not Ali who was also killed? Why specifically Hussein only? It's because Ali himself was killed by Al Khawaarij, who were part of the Shi'a of Ali during the battles of Al-Jamal (the camel) and Siffeen.
It's quite obvious that ashurah is specifically done every year to make the people of Sunnah look bad. Because Yazid was not shi'i, the shi'a make it look as if the people of Sunnah are bad for the death and martyrdom of Hussein. If they really cared for the ahlel bayt, wouldn't they be grieving everyday for the death of all the pious heroes of islam throughout our islamic history on the days each of them died?
Here's a link which discusses the issue more in depth;
http://www.ahlelbayt.com/articles/matam/no-ashura
I hope you understand from that that ashoora is a sad day, but wasn't it the shi'a themselves who actually became cowards and never supported Hussein - which actually lead to his death? The only way we can truly show our love for him and the Prophets family is by obeying the Messenger of Allah and what was revealed to him.
And whoso obeys Allah and the Messenger (Muhammad SAW), then they will be in the company of those on whom Allah has bestowed His Grace, of the Prophets, the Siddiqun (those followers of the Prophets who were first and foremost to believe in them), the martyrs, and the righteous. And how excellent these companions are!
[Qur'an 4: 69]
Fasting on ashoora is something which will be rewardable;
http://islamqa.com/index.php?pg=rslt...glist=0&ln=eng
And Allah knows best.