Each Royal House has her own protocol, also regarding the tiaras. I example, in Royal House of Denmark the royal fiancee has the right to wear a tiara in the eve of her wedding (or may some days before the wedding). So CP Mary wore a tiara in the Opera in the celebration the day before her wedding.
On the opposite side, Royal House of Spain, CPrincess Letizia, could not wear a Tiara before her marriage so the firts day she wore a tiara it was IN the day of her wedding NOT before. You see her in the Danish Royal wedding, one week before her own wedding and also in the eve of her wedding in the Pardo Palace reception, with the stunning grey-silver dress, she did not wear a tiara. Since her wedding she regularly wears tiaras.
The other question is . When the protocol allows you to wear a tiara, are you OBLIGED to do it???? NO IDEA