Umayyad celebrate the day of Ashura as festival
Bani Umayyah (the cursed tree) forced people to celebrate the day of Ashura (10th of Muharram) as a blessed day, and encouraged people to buy and store their annual food on that day and to fast that day as Sunnah, by fabricating traditions in this regard which were solely designed to overshadow and to cover up the great catastrophe of the slaughtering the household of the Holy Prophet (SAWA).
To make people believe that Ashura is a blessed day, Bani Umayyah fabricated that it was in that day when Allah saved Bani Israel from Pharaoh. This is while Moses (AS) and his companions were saved and passed the Nile river in the Month of Rabiul-Awwal.
They also invented that the repentance of Adam was accepted on that day, While it was accepted in the month of Zil-Hajjah. They say that it was the day that Jonah (Yunos) was saved from the body of fish and the ark of Noah Was settled on earth while it was on the 18th of Zil-Hajjah. All those Traditions were fabricated By Bani Umayyah to cover their crimes.
Ashura is the day of mourning and lamentation. In a tradition from Ahlul-Bayt, it is said that \"whoever celebrates this day as a blessed day, he Will be resurrected in the day of judgment with Yazid.\"
The reason is Clear, and it is because there are traditions from the Holy Prophet (SAWA), everybody who is pleased with an action, he will be resurrected in the day of Judgment with the one who did that action. Performing a non-obligatory fasting is a sign of being pleased and a sign of happiness.
Such traditions are narrated in Sunni Hadith. But remember that there are many contradictory reports are within these Hadith which disprove your hypothesis as being Sahih hadith.