Knowledge is power.
But these days people are having different opinions and they may feel that there is no merit in these beliefs. So it all depends on the individual thinking and beliefs. However, marriage should not be performed for at least 11 days. after the death.
If at all it is the case that in the somber mood of the death the marriage needs to be postponed, then, if possible, the couple could have a registered marriage on the already decided date, and at a later appropriate day have the marriage with the traditional rituals. Alternatively, go ahead with the marriage on a smaller scale. Just have only both the immediate families present without other relatives or friends.
Death is a part of life. Let not a happy couple's marriage get bogged down by traditions.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell