A wedding is a public situation. In certain rare cases you actually could use “declare,” but in that case the meaning would be more focused on their decision to get married rather than the wedding. Most of the time, announcing a wedding is about a public event that people will attend. After years of fighting with her parents about her boyfriend, Julia declared over dinner that they were getting married. (Focuses on her decision to stay with him over listening her parents) They announced their wedding on Facebook. (focuses on a public event) Side note: what you send out before a wedding is called a wedding invitation. What you send out after a wedding to tell people you got married is called a wedding announcement.