Must I give Sadaqah, and how do I give it, knowing that I did not give Sadaqah on Ashura due to my ignorance of it?

One who possesses the nisab (definition) (minimum taxable amount) of wealth and then uses it to purchase trade goods must pay zakat (definition). Zakat is obligatory on both the principal capital and the profits together upon the completion of a hawl (lunar year) from the time the original capital was acquired, reckoned by the Hijri months. The goods are valued in cash, and one-quarter of a tenth (2.5%) is to be given, after deducting outstanding debts owed by him and adding debts that are expected to be collected. If the remaining amount reaches the nisab (equivalent to 85 grams of gold), then zakat is due; otherwise, it is not. It is not permissible to delay the payment of zakat except with a valid excuse.