It is not permissible for you to wear fancy clothes or anything else on Aashooraa because that may make ignorant people or those with ulterior motives think that the Sunnis are happy about the killing of al-Husayn ibn Ali (may Allaah be pleased with them both), Allaah forbid that that happened with the approval of the Sunnis.
With regard to your interactions with them by backbiting about them and praying against them, and other kinds of actions which are indicative of hatred, this serves no purpose. What we have to do is to strive to call them (to Islam) and to influence them and reform them. If a person is unable to do that, then he or she should turn away from them and leave the dawah activities for those who are able to do them, and not do anything that may put obstacles in the path of dawah.
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said:
Because of the killing of al-Husayn, the Shaytaan started to spread two kinds of bidah (innovation) amongst the people: the bidah of grieving and wailing on the Day of Aashooraa, striking the cheeks, screaming, weeping and reciting eulogies ; and the bidah of expressing happiness and joy. So some express grief and others express joy, so they started to like the idea of wearing kohl, taking a bath, spending on their families and preparing special kinds of food on the day of Aashooraa and every bidah is a going astray; none of the four imams of the Muslims or others approved of either of these things (either expressing grief or expressing joy)
(Minhaaj al-Sunnah, 4/554-556).
Reproduced from Islam QA