For me the saddest part in My name is Luna was when she thought they forget about her birthday. When she apologized for bothering them it breaks my heart.
While that scene is sad in a way, for me the following 2 scenes eclipses that scene:
1) Sofia's death & the effect it has on Luna's group
The way Sofia died was........... the most haunting scene out of that entire flashback. As if being malnourished and mind being broken and hallucinating wasn't terrible enough, she dies through electrocution
.
And Luna trying to emulate the hands heart scene that she used to do with Sofia when she was alive, but instead of Sofia's physical hand, the scene shows the "ghost" hand of Sofia matching Luna's hand to shape the heart....... man that was absolutely heartbreaking.
2) Gaia gets "broken" & betrays Luna
Second saddest scene for me. That scene where Gaia gets completely broken, becomes cuckoo crazy and starts hurting Luna so she can survive was.... heart-rending.
Gaia was the pillar that pretty much everyone relied upon and was welcoming to Luna even from early on (which Luna came to rely upon her as Big sis in a way). To see Gaia be turned against her like that, just adds to the whole tragedy of it all (though one saving grace of it was Viola coming to Luna's rescue, standing up to Gaia)
If any one of or both of the above two scenes I mentioned above didn't exist in My name is Luna, I don't think the event would have felt anywhere near as impactful for me as it does.
Those 2 scenes literally pulled at my heartstrings.