The Bold and the Beautiful has been revolving around the storylines of the Forrester family, especially Steffy Forrester and Finn Finnegan's marriage. After Finn finds out that he is Luna Nozawa's biological father, the storylines indicate that things might go wrong very soon.
Disclaimer: This article contains spoilers for The Bold and the Beautiful and may have speculative elements.
Steffy might likely find out about this situation on the day of her anniversary with Finn. Fans of the soap took to Facebook to discuss this possibility:

In the comments of the post, fans of the soap commented about how this might be an extremely unexpected anniversary gift for the character:

Many viewers also seemed to believe that Steffy deserved what was coming to her and that the truth about Luna was karma for her:

While a few fans seemed to be a little sympathetic towards Steffy, the common consensus seemed to be that fans are excited about the revelation and to see what happens:

What is the shocking truth that Steffy might have to hear on her anniversary on The Bold and the Beautiful?

As per the current storyline on the soap, Dr. Finn Finnegan has been faced with a revelation that changed his life dramatically. Finn found out that when he slept with his former flame, Poppy Nozawa, it increased the likelihood of him being her daughter Luna's father.
Finn was taken aback while trying to process all this, even remembering that Luna was the one responsible for kidnapping and tormenting his wife Steffy Forrester. Poppy then spoke to Finn and apologized to him for keeping Luna's parentage a secret from him, confirming Finn's worst fears.

Luna's paternity has been an important aspect of the storyline for many months, with many characters' fates depending on it. For Luna, she might find out this information and decide to put Steffy and Finn in harm's way. On the other hand, Steffy finding out about this might pose great strain on her marriage with Finn.
Now spoilers hint that Steffy might get to know this truth on her anniversary with Finn. It remains to be seen how Steffy reacts to the truth about Finn being Luna's biological father, given the family's checkered history with Luna already.
Viewers can watch The Bold and the Beautiful on Paramount+ and CBS, with new episodes released on weekdays via these platforms.