Thank you for sharing this beautiful ritual, I read that in the past it took about 3 days and nights but now lasts a day. As it is a ritual that comforts the souls of deceased to comfort and guide them to the Buddhist land of bliss.

Also I have read that due the Sewol Ferry Disaster with 325 students of age 16-17, it is a remembering day too.

I believe those nuns probably pray for those souls. That is really important fest and it is amazing that born during Goryeo period still exist and carried out.

Cheers, @Stef1