If this is the voluntary fasting you can go ahead but if she is fasting her missed days from ramadan you are not allowed to approach her.

If a wife is fasting and husband wants to be intimate, am I allowed to go ahead as some people say ‘you are’ and some say ‘no’. Please advice me.

Fasting | February 21, 2014 1:57 am
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Suppose I broke an oath, but I am too shy to tell my parents to feed or cloth 10 poor persons on my behalf. In this case can I resort to fasting to fulfill the expiation?
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I wanted your expert advice on fasting during the month of Rabi-ul-Awwal is there any instance from Sahaba or Tabieen or tab e tabieen for fasting in this month?