You should not observe voluntary fasts when you still owe one or more days from Ramadaan, rather you should start with the fasts that you owe from Ramadaan, and then observe voluntary fasts.
If you fast the tenth and eleventh of Muharram with the intention of making up days that you owe from Ramadaan, that is permissible and will make up for two of the days that you owe. The Prophet (peace and blessings of Allaah be upon him) said, “Actions are judged but by intentions, and every person will have but that which he intended.”
Fataawa al-Lajnah al-Daa’imah, 11/401
We hope that you will have the reward for making up the missed fast and the reward for fasting that day.
Fataawa Manaar al-Islam, by Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him), 2/358.
Praise be to Allaah.
If we do not know whether the month of Dhu’l-Hijjah is complete (30 days) or imcomplete (29 days), and no one tells us when the new moon of Muharram was sighted, then we should follow the basic principle, which is that the month should be completed as thirty days. So Dhu’l-Hijjah is counted as being 30 days long, then we calculate the date of ‘Aashooraa’ on that basis.
If a Muslim wants to be sure that he has fasted on the right day, he should fast two consecutive days at ‘Aashooraa’. So he should calculate when ‘Aa’shooraa’ will be if Dhu’l-Hijjah is twenty-nine days and if it is thirty days, and fast these two days. Thus he will be definite that he has fasted ‘Aashooraa’, and in this case he will have fasted either the ninth and tenth, or the tenth and eleventh, both of which are good. If he wants to be sure of fasting Taasoo’ah (the ninth of Muharram) as well, then he should fast the two days we have spoken of above and the day immediately before them as well. Then he will have fasted the ninth, tenth and eleventh, or the eighth, ninth and tenth. In either case he will have fasted the ninth and tenth for sure.
If someone were to say “My work and other circumstances only allow me to fast one day, so which is the best day for me to fast?” we say to him:
Count Dhu’l-Hijjah as being thirty days and work out the tenth day (of Muharram), then fast on that day.
This is a summary of what I heard our shaykh, ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him), say when I asked him about this matter.
If we receive news from a trustworthy Muslim that he has seen the new moon of Muharram, we should act in accordance with that. Fasting at any time in Muharram is Sunnah, because the Prophet (peace and blessings of Allaah be upon him) said: “The best fasting after the month of Ramadaan is fasting in the month of Muharram.” (Narrated by Muslim, 1163)
And Allaah knows best.
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).