The twins have invented a new word: lasterday.
Lasterday is a cross between "last night" and "yesterday" and it appear to be a synonym for "the other day" when we use that phrase to mean some span of time in the past that could be anywhere from one week to 20 years go.
For example: "Lasterday, when the power was out, we had to use our flashlights."
Or: "Lasterday, when I was in college, you could ride the subway for 60 cents."
song: Yesterday • artist: The Beatles