Okay..here I go...
Part I
the poornima thing is actually related to the eldest son in the Ranawat family , Swayam.Swayam's existence has been kept hidden from the outside world as he suffers from a split personality.During daytime,he becomes the extremely violent Swayam , and durinf the evening , he becomes a dignified prostitute , who goes by the name of Payal.Swayam's condition is due to the fact that he fell head over heels in love with a tawaif Payal , but his marriage to her was prohibhited by his granddad and dad .His dad tricked him that Payal loved him only for money and hence ,Swayam's mental state deteriorated.
When Aastha got married to Shubh,nobody told her about this secret.But Aastha finds out about Swayam and helps the family members to feed him , etc,While Shubh takes care of Swayam in the day , Aastha takes care of him in the night , ie,she takes care of Payal.
In the course , Aastha discovers that the girl Payal was actually a tawaif .She and Shubh go to the area where Payal used to work and find her diary.Then both of them come to know that Payal had never betrayed Swayam , but it was all a trick by Swayam's dad.
Aastha and Shubh manage to heal Swayam , but then the real truth comes out - Payal and Aastha are lookalikes , and thats why Shubh's dad agreed to the Shubh-Aastha wedding.Swayam mistakes Aastha for Payal ,but Sulochana and Vanraj (Shubh's mom and dad) dont want to tell Swayam the truth so quickly , because they fear that Swayam will go mad once again.
Swayam prepares for his suhaag raat with Payal , but Aastha tells him that she has taken a vow that she will not consumnate her marriage to him until Navratri , and so Aastha remains faithful to Shubh.
From then on , Astha-Shubh are torn between their duty and their love for each other-Aastha Shubh cant have any loving moments together , as Swayam will think that Payal is having an affair with Shubh.