What is the ruling on fasting a day in gratitude to Allah? And is it obligatory or recommendable?

Gratitude is expressed through belief, word, and deed. There are acts of worship and devotions prescribed by Allah Almighty as a way of showing gratitude for His blessings. These include: prostration of gratitude (sujud al-shukr) upon the renewal of a blessing or the warding off of an affliction; the night vigil prayer (Salat al-Layl) as performed by the Prophet, peace and blessings be upon him, in gratitude to Allah; fasting on Ashura in gratitude to Allah for saving Moses; zakat al-fitr in gratitude for completing Ramadan; the pilgrim's offering of the tamattu' sacrifice as gratitude to Allah for combining two rituals; and the 'aqiqah upon the birth of a child as gratitude to Allah.

Gratitude may also be expressed through other acts of worship, such as Ka'b ibn Malik and Abu Lubabah giving away their wealth in charity after their repentance was accepted, and Abu Hurayrah freeing a slave in gratitude for reaching the Prophet, peace and blessings be upon him. These acts of worship are recommended, not obligatory.