This is harder to answer than other questions because I can think of a million moments that she was incredibly happy and none of them are less important than another. I can’t seem to pick one. The happiest day before she became a vampire was possibly one of her performances on the stage. Gabriel made her incredibly happy after they got together but it was not a happy day when they first met… His presence was a source of happiness for her back then, so I can’t pick a day with him and claim that she was much happier on that day than the day before. I didn’t think their time together throughly so I can’t come up with a single occasion as an answer for this question. 

On second thought, her happiest day before Gabriel died was the day she and Maion went to Vienna together, actually (which is something I haven’t realized until now). She was always happy when she was with Gabriel but at that Winter Ball, she was incredibly amazed and brightened by Maion’s presence and the fact that he asked her to come with him. Wow, ok. I learned something. Moving on.

She spent almost a century by his side and of course he made her happy in every possible way but I guess I can think of two days/nights that she was ridiculously happy, both includes some kind of ‘level-up’ in her relationship with Maion.

First one is the night he gave her the jasmine ring - she finally manned up and told him that she loved him. The second one was a bit dramatic; Leopold’s illusions messed up the place, she and Maion had a fight and she went a little crazy after Maion sent her out of the room, not believing a thing she said… Then things happened and Maion shared something with her that he only shared with Rodrigo until that day. It would take pages if I started to explain in detail but yes, it was le big deal.