If there is a da’wat (e.g. nikah da’wat) on ashura’ day should one fast or attend the da’wat.
Bismillahi Ta’ala
Walaikum Assalam Warahmatullah
Although fasting on the day of Ashura is optional, however the merits and virtues of fasting this day are manifold as noted in many ahadith.
One of its greatest merits is the expiation of minor sins for the past year. Indeed, neglecting this fast would mean missing on its significant reward. As Muslims, it beehoves us not to disregard such a reward.
It’s understood that answering the invitation holds its own merits, however, comparing these merits to the immense rewards of fasting on this day, it’s evident that copious rewards of Ashura fast far outweigh these.
My advice is to fast this day, and if possible, still make an appearance at the nikah ceremony as a good-will gesture to the host and his invitation and potentially inspire others to fast on this day in the future as well. The host should be empathetic to the fact that, as a Muslim, one wouldn’t willingly want to forego this fast.
Once you have started your optional fast, it becomes binding on you. If for some reason you end up breaking your fast at the ceremony, then you will need to keep a Qadha’ to make up this fast.
Wallahu A’lam
And Allah Ta’āla Knows Best
Mufti Faisal al-Mahmudi