Under what circumstances may I attend the wedding and party of my best friend’s son during the year of mourning for my father?

In general, one should avoid weddings during the year of mourning even of close friends and relatives. If one's absence will cause great unhappiness to the couple, one should try to attend an aspect of the wedding at which there is no music. If this is also impossible, one should assist with the wedding preparations on the day of the affair. It would also be correct to abbreviate one's participation (e.g. only attend the Chuppah or leave the affair a bit early. A mourner should not join in the dancing.