The correct answer is the date the marriage is considered legal. In some countries, it is when the marriage paperwork is signed. In other countries, it is when the marriage is registered with local officials.
Example: Wife and I had our "ceremony" on July 27th. My wife, the two required witnesses, and I signed our marriage contract on that day. However, we registered our marriage with our local Household Registration Office on July 30. By Taiwanese law, July 30th is our official date of marriage.