This Valentine’s Day project’s theme is Proposals. Here is a picture of the day Diamond Tiara proposed to Spike, Sweetie Belle, and Pipsqueak 
Poly relationships are fairly common in Equestria, as is marrying multiple partners. Of course, Diamond herself didn’t know that when she first started feeling romantic to two ponies and a dragon. A few months and a minor breakdown later, the four of them were adorable together.  
Diamond crafted the Engagement Jewelry herself; a ring for Sweetie(since she’s a Unicorn), an earring for Pip(easy to hide in his theatre costumes), a necklace for Spike, and a tiara for herself, of course.  
The four of them had been in a (rewritten) Hearths’ Warming Eve play. When everyone sang the carol at the end, Diamond sneaked backstage, took off her costume, and grabbed the jewelry box. Of course, everypony else was in on what would happen next.  
As everyone began to walk off stage, Diamond asked her three partners to stay for an extra part. They were confused, but agreed. She gave a short speech about how much she loved the three of them, how much they meant to her, before asking them to marry her.