Praise be to Allah, the Creator and Cherisher of the worlds and peace be upon his prophet Muhammad SAW. The blessed month of Ramadan is near completion. A month which brings mercy, forgiveness, and freedom from fire if completed virtuously. The month of Shawwal, which follows Ramadan, also has unique opportunities to continue to earn Allah’s blessings. Muhammad SAW said, “The one who fasts the month of Ramadan, followed by six days in Shawwal, is purified of sins as if he was just born.” (Hadith: Sahih Muslim and Abu Da’ud).
The six days of fasting in Shawwal can begin any day after Eid al-Fitr and can be completed continuously or spread throughout the month. There are differences of opinion regarding the requirement to make up missed Ramadan (fard) fasts before keeping Shawwal (nafl) fasts. See the link below from Yasir Qadhi:https://m.facebook.com/yasir.qadhi/posts/q-shaykh-i-have-to-make-up-some-fasts-of-ramadan-but-i-also-want-to-fast-the-six/10152391208338300/
However we choose to do it, certainly, rewards come from Allah and anything we do sincerely to please him will not be wasted.
As for those who believe and do good deeds, rest assured that We do not waste the reward of him who does a good work. (Quran 18:30)