Frozen 2 starts out with a flashback on Anna and Elsa's parents, much like the original movie, and it's actually set on the same night as Frozen's. The original Frozen was released in 2013 and became an instant hit for Disney. The sequel released in 2019, and subsequently Frozen 2 was added to Disney+ early in the wake of the coronavirus
Frozen followed Elsa's journey of self-discovery, and how she learned to control her powers without sealing herself away. Frozen 2 picks up a few years after the first movie, with Anna and Elsa living happily in Arendelle. After Elsa hears the call of the "unknown,", she and Anna explore the wider world to find out the source of Elsa's spiritual powers. When the sisters venture into the Enchanted Forest, they stumble upon the people trapped inside and learn more about the death of their parents.
Anna and Elsa's parents are primarily seen in the flashback scenes that start both Frozen and Frozen 2. In the first movie, after Elsa accidentally hurts her sister, the parents take Anna away to be healed by the trolls, and she also has her memories of Elsa's powers erased. Frozen 2, meanwhile, starts with a flashback scene involving the King and Queen telling their children the story of the Enchanted Forest. In both of these scenes, the parents and children are all wearing the same clothes. The only change in appearance between these two flashback sequences is the color of Agnarr's hair, which appears much blonder in Frozen 2. The fact all these characters are wearing the same outfits puts these two scenes in place alongside each other.
Beyond the same clothes, the narrative flow of these sequences makes a lot of sense. In Frozen 2, Anna and Elsa are put to bed after playing together with Elsa's snow toys. While Anna definitely passes out, the movie never shows Elsa falling asleep. Then, in Frozen, Anna wakes up Elsa so the two can keep building snowmen. If Anna fell asleep far earlier than Elsa, it makes sense that later in the night, Anna would wake up and ask Elsa if she wants to build a snowman. Plus, it helps establish how little time Anna and Elsa spent together before Anna's memories were altered. There's a lot of evidence that arises once the clothing connection is made.
A lot of Frozen 2 revolves around reconnecting with the past, so it's not a reach to assume that this scene connects to the flashbacks of the first Frozen. Not only is it a touching moment about the sisters' past, but it's a perfect way to start off the sequel. The fact Frozen 2's flashback seems to link to the previous film's flashbacks makes the movie stronger. If Disney ever makes a Frozen 3, hopefully any new flashback scenes will go together as well as these do.