Making up for not fasting optional days
23-7-2012 | IslamWeb
Question:
Is it permissible to make up for fasting the white days? Will this be rewarded with the same reward as fasting on them? Should there be an intention?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
If someone is used to fasting on the White Days (the 13th, 14th, and 15th of every lunar month) and missed fasting them, he may make up for them and will attain the same reward of fasting on them, especially if he missed them with an excuse. There should be an intention when fasting them and when making up for them. Imaam An-Nawawi 
said, "Specifying an intention is a condition for the optional fasting like fasting on the day of 'Arafah, the day of 'Aashooraa', the White Days and the six days of Shawwaal just as it is a condition in regular voluntary prayers." [End of quote]
Allaah Knows best.