The traditional recommendation is to fast six voluntary days during Shawwal after Ramadan.
Shawwal is the tenth month of the Islamic lunar calendar, immediately following Ramadan. Its first day is Eid al-Fitr, when fasting is prohibited, so the optional fasts begin afterward. They may generally be observed on any six days of Shawwal rather than necessarily in one uninterrupted block, although views about consecutive fasting vary among scholars and traditions.
The practice is linked to a hadith recorded in Sahih Muslim: fasting Ramadan and then six days of Shawwal is described as carrying the reward of fasting for the whole year. The familiar explanation uses the Islamic principle that a good deed receives tenfold reward: thirty Ramadan days plus six Shawwal days totals 360 rewarded days, approximately a lunar year.
These fasts are voluntary, not a replacement for obligatory Ramadan days. People who owe qada fasts may encounter differing scholarly guidance about whether to complete those first or combine intentions, so the answer should not be presented as a universal legal ruling.