ANSWER:

You can intend voluntary fast any time providing you didn’t consume anything after Fajr. 

when is the latest you can make intention to fast a voluntary fast? E.g after fajr Adhan but before the white thread of dawn?

Fasting | December 12, 2017 5:12 am
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It was said by the prophet (S.A.W) that fasting on yaohm jumu’ah and sabt is prohibited except on itsnain and khamiz, it came from another hadeeth that it is sunnah to fast on 13,14 and 15 of lunar month. Here is my question,what if this three days falls under the days we are not allowed fast, how can we treat this hadeeth?
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If a woman is in the state of impurity (periods or janabat) gets a bath to purify herself after fajr prayer let’s say by 11 a.m. but she has not eaten anything after fajar. So can she intends to observe fasting for that day when she gets purified after fajr salah?