All right, let's start with the purpose here. I don't know if you were writing to a specific style but given the length I'll assume traditional flash fiction. Now, the purpose... to me is not entirely clear. GotWala says it's a real message, but I don't see one. In fact, I don't think we even have a clear conflict or character arc here. Purpose aside, the conflict/character arc is somewhat universal, so let's start there.
So character arc is kind of important. By its very definition, a protagonist has to change throughout a story. The story is then a narrative frame for how that character changes. It doesn't have to be good, or bad, but it should be linked to the central conflict, and it should be tangible. The motivation and change of the character is what keeps a story interesting.
So how does Nona stack up? Well, in the first place in order to have clear growth, one has to have a clear starting point. We need to know who Nona is at the beginning of the story so that we can compare her to how she is at the end.
Nona's narration is very distant; she has to manually decide to smile. She thinks about how she's supposed to act rather than how she actually does. We see a character who is stuck inside her own head, and just about as detached from the world around her as a character can practically be. This is very well done for establishing this kind of character trait.
Now we get down to the line ...however, my eye doesn't catch my knight in shining armor on one knee. Once again, she is describing her senses rather than experiencing them as part of them, but this 'knight in shining armor' business... I can't tell if she's being facetious or what. On one hand, she consistently sounds pretty ambivalent to this guy, if not annoyed by him, by the Again near the start. After this line, I am completely confused as to how we're supposed to understand the relationship between her and David.
This upcoming line does little to help that:
I lied, flawlessly shelving my disappointment of there being a reservation ...or was I even upset at all? Whatever, my pain wasn't important at the moment..
So, on one hand she's disappointed enough to call it pain, but on the other she's not upset? This doesn't make a whole lot of sense.
Then end in the end, we have some kind of dawning moment that seems to break through Nona's complete and utter detachment. This has to be the emotional climax, when she realized the mother allowed the ring to be passed onto her. So ideally, it should be at this point that the character changes. For better, for worse, but it needs to be a visible change. What is the effect this episode had on this person's life?
I don't see anything clearly to answer that question right now, but if I had to pick something out I would say that it makes her a little bit less distant? On one hand the closing Touche is a fairly pointed and involved remark. That's the most energy we've seen her express in the entire writing, which for an ending note, is a good thing. On the other hand, we continue to see her distantly deliberating on her action in I had to allow him to ramble while I decided if I was really sorry for his loss. Contrast this against a slight rephrasing such as, "I allowed him to ramble as I decided if I was really sorry for his loss." In the first, she continues to act out of obligation-- nothing has changed. In the latter, she is acting clearly of herself.'
So in summation I don't think I can say we have a clear character arc. We start with a detached girl, with a questionable relationship, who has a revelation about this bitchy old woman... and then continues to be somewhat detached.
We can do better. Allow me to take some creative liberties here for the sake of example.
Let's say our goal is to make this a well-defined arc. We'll start with a girl who's distant. She'll get a ring indirectly from a woman who hated her, which will cause her to change to be more engaged with the world around her. I'm going to continue this critique operating under that as a presumed goal.
Now, as much as it is a result of the climax that changes the protagonist, the climax only occurs because of a certain conflict. Therefore the conflict needs to lead the character to that change. This works out nicely because conflict, along with liking a character, is the best way to get a reader involved in a story.
So here's our problem, and I alluded to it earlier: you don't actually introduce any tangible conflict until about a third of the way through your text's length. I would say the it isn't until the line ...however, my eye doesn't catch my knight in shining armor on one knee that we as readers actually know for a fact that something is wrong. We get the impression of it before from Nona's general attitude, but this is where it becomes clear.
...The thing is, if you wait until a third of the way in to introduce your conflict, you don't have as much time to develop it. More important, conflict is the hook. You need that conflict immediately to keep readers interested. On the internet most people won't read more than a sentence if they aren't immediately engaged. If you get into more traditional writing fields then it's a market necessity. You have to grab reads right away.
So let's look at your opening paragraph.
We get the rain, we get the day, we get a taste of the setting and culture, and we are told that the narrator is not alone when she usually is.
To be frank, none of these things are particularly commanding. In fact, the rain, and the common lack thereof may as well be completely worthless; they aren't relevant to the story after this. In flash fiction, every word counts; there's no room for useless details. Even in any fiction or writing, the same is true for the opening. Capture the reader immediately; you don't have time to waste describing running out into the street-- if that's important, which it doesn't seem to be-- then it can come later.
The take-away is this: Start with the conflict. It's your best chance for a hook. In this case, let's say that in a sentence you're able to pain the picture of a guy presenting a ring to a girl without proposing. Right there, we know something's weird. That's all it takes to make a conflict. If you start with that scene, you're going to be able string a whole lot more people along.
So we have an arc for structure, and we've set our conflict at the start and our reader is interested. Now that you've got them hooked, you can tease them a bit. Step back and give them whatever exposition is necessary. I still would not include the details about the rain or standing in it. They don't benefit the story, but this is where you put the things they need to know. For instance, you might introduce it as Christmas Eve still (which, by the way, does not actually seem like a good choice either-- proposing on Christmas Eve may be cute and all but it subverts the intensity of the situation. Getting a felt box on Christmas could still be just a Christmas present; getting a felt box on September 26th or whatever has a lot more immediate pressure to it) or describe the revelation of the said box leading up to the point of the opening disconnect.
Or, even better, trail into some story about David's mom immediately. Let's take a look at your apparent climax.
So David drops this huge bomb that OH this person just died this morning WOW.
For one thing I question why he continues to choose that day to propose. That seems a little odd.
More importantly, so he drops this bomb, and then the action comes to a screeching halt as you have to stop the action and explain what kind of mess the mother was. This is a huge speed bump in your mood.
So now we have a convenient solution. So you have a window to provide your exposition in, and you have exposition that needs to be provided. Let us learn about the mother well before we need to. Put that gun on the table so we can fire it later. Let the tension in the story develop so that we as an audience feel this looming threat of the mother's disdain throughout the text so that, when we learn she's dead, we at once celebrate with Nona, and are taken aback that she would do something that we've come to learn is clearly out of character with her.
At that point, this moment will feel impactful enough that we believe it changes Nona. Follow through and show us that change, and then this will feel like a cohesive narrative and worthwhile story. There is a bit more I could say, but the least, there are some key points to keep this, or any future work, on task and it is important to iron out the structure before building in the details.