Kristoff invites them to a Halloween party, because why not? Elsa's excited because she likes Kristoff; plus, a social setting with Anna would be nice. She's excited to get out of her studies, and Elsa can't blame her. She looks so tired all the time.
Regardless, she still comes home with a smile, or a hug, or a touch to the shoulder. Their lives are almost domestic, which is what makes it painful. Maybe she doesn't even notice.
So, Elsa says nothing. Even if her bed is still too large, in desperate need of some warmth.