One can fast three days of every month and this reward is the same of six of shawwal.

Kindly refer to the following link(s):

https://www.youtube.com/watch?v=QHPEgSZwMxU&t=9s

If someone wishes to fast the 6 days of shawwal but cannot fit them in, can they fast after shawwal those fasts or do they lose them?

Fasting | July 19, 2017 10:55 pm
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I know that we have to make up our missed days of ramadan and then fast the 6 days of shawal. My sister doesn’t have time to fast first her missed days and then the 6. That is why she asked if she can fast the 6 days first before shawal ends and then her missed days. Is it permissible?
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I got my menses just 10 minutes before maghrib. Is my fast valid?