Yuuka is the flower master of the four seasons. Because she takes her title very seriously, she has taken it upon herself to relegate the four seasons. When its time for a season to end, she buries its respective touhou until its their time once more.
Here, because winter is over or will soon be over, Yuuka is releasing Lily from her tomb unto the world. Lily is so overjoyed to see the light once more that she's on the verge of tears.
...judging from the date on Pixiv and seeing that Yuuka is mostly associated with Summer... I think you meant to say, Yuuka is sealing her away as her season is in the process of ending. A few more rocks (weeks) and Lily won't be heard from ever again...
Well, it was before translation and where I live we either get rain or shine only. No four seasons here so I wouldn't even be able to tell if its summer season without looking it up.
And "The light! How I missed the light!" was funnier in my head than the burying alive scenario.
