When one performs sunna fast, is it permissible to attend a wedding?

The validity of a sunna fast does not depend on one’s whereabouts during the fast. A valid fast is fulfilled by two conditions (Reliance of the Traveller, i1.13):

a) the intention

b) and refraining from things which break the fast.

Attending a wedding is not one of the things that invalidate a fast (see The Reliance of the Traveller, i1.19).

However, one should be careful of attending an event that might incur the displeasure of Allah, Most High, especially while one is trying to gain closeness to Allah through this sunna act of worship. If a wedding consists of impermissible mixing with the opposite sex, impermissible dancing or music, one must avoid it.

The following answer is very useful and could apply to the questioner’s situation, if the wedding consists of haram actions:

Maintaining ties of kinship: Interaction with relatives who are not cautious in mixing with non-mahrams:

As for a wedding that does not consist of anything haram, it is permissible to attend it while fasting.

And Allah knows best.

Shazia