Vow

If you made an intention to fast on a specific day as a vow or Nathr نذر then you must fast on that specific day even if you did not state it verbally. If your intention was to fast after the event, then you are not bound to fast but after the event. If your intention was for fast anyway with out a condition, then you can fast any time before or after the event.

Wassalam.

Fulfilling the Nadhr نذر (vow) is compulsory. You must fulfill whatever you remember.

Wassalam.

Vow نذر when you commit to do a worship like fasting or praying recommended Prayers like Night prayer, or good act like helping needy or give something in good cause if you get your wish. 

Oath يمين is when you take oath to do something good or leave a bad deed like taking oath to leave smoking or leave attending mixed gatherings etc.

Covenant عهد is when you promise Allah (SWT) to do something good or leave something bad.
Fulfillment of vow, oath and covenant is obligatory.

If you don't remember what intention you had, you need to just fulfill what you had committed no matter what was whether vow or oath or covenant as all of them share a commitment which must be fulfilled. Failing to fulfill the commitment is a sinful act.

Wassalam.

Making a False statement by Qasam or oath or saying Wallah, is a major sin. It is called in the Hadeeth ( Al-Yameen al-Faajirah Al-Ghamoos) (The sinful oath which immerses the liar in the fire).

Seeking forgiveness needs: 1. Giving back the right of any one who was been wronged by this false statements and making him pardon you from his heart.

2. Repenting and feeling sad for what you committed.

3. Firm decision not to do it again.

 4. Seeking forgiveness from Allah (SWT) day and night.

Wassalam.

Believing that every good is from Allah and every wish is granted by Allah, seeking from Ahlul Bayt (AS) their intercession to Allah to grant us our wishes is very good.

'Wassalam.

The verse 89 in Sura maidah does not permit deliberate lying. [Allah will not impose blame upon you for what is meaningless in your oaths,but he Will impose blame upon you for breaking what you intended of oaths.].

wasalam.

You should feed ten poor persons or give them clothes. If you can not afford that, then you must fast three day. It is called Kaffara al-Yameen.

'Wassalam.

The Qasam which is obligatory to fulfill is when you take the name of Allah in it. If you did not take the name of Allah, then such Qasam is not obligatory and no Kaffara  on you if you do not fulfill it.

Wassalam.

It is obligatory to fulfill the promise to Allah, and if you did not fulfill it, you will be committing a sinful act and there is a Kaffara on you which is either, freeing a slave, or feeding sixty poor persons or fasting two months.

We are not allowed to change our mind after promising Allah.

The Promise should be pronounced verbally and not just in your heart.

Wassalam.