From past to present: When Aarti was holding the sieve to the moon, we could witness Prashant through it, behind Aarti. Both Prashant and Yash were behind her, but Aarti ends up turning towards Yash, praying for his well-being.
The point of Karwachauth: As soon as Prashant cannot be seen through the sieve (when Aarti turns to face Yash), he immediately feels pain in the stomach. This is because Aarti has stopped praying for the longevity of his life, but has shifted her focus on Yash instead. This might be considered foreshadowing of his death in the future.
Yash's vrath for Aarti: I don't think Yash ever kept a Karwachauth vrath (fast) for Arpita. But, he's keeping one for Aarti. This foreshadows that Aarti will be okay, this pregnancy won't kill her, nor will anything else.
Protecting the diya: Aarti places her hand in front of the sieve and Yash places it behind it. The front represents Aarti's husband, Yash, while the back represents Prashant. Aarti will do anything to protect Yash (as we've seen before too), while Yash will do anything to protect Aarti. This is where Yash's presence takes over Prashant's. Prashant was once obligated to protect his wife, but now Yash is taking the responsibility of her (and her past) for the rest of his life.
There was beautiful symbolism in today's episode!