It is highly recommended to fast six days of the month of Shawwal, as this is a meritorious act which results in abundant reward. However, it is not required to fast six days continuously without any interruption. One can fast according to convenience any time during the month.
As reported in Hadith in Muslim:
Abu Ayyub al-Ansari (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.
The Prophet (peace and blessings be upon him) is reported to have said, “Whoever fasts the month of Ramadan, and then follows it up by (fasting) six days of Shawwal, it is as though he has fasted the whole year.”
As reported in Hadith in Muslim:
Abu Ayyub al-Ansari (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.
The Prophet (peace and blessings be upon him) is reported to have said, “Whoever fasts the month of Ramadan, and then follows it up by (fasting) six days of Shawwal, it is as though he has fasted the whole year.”
