Navigate the date
Moving the date lets you see what you'll have to spend on a day that hasn't arrived yet.
The date sits at the top of the Budget screen. Swipe left on it to move forward a day, right to go back, and everything follows: the coins, the peek card, the transaction list.
Moving forward is the interesting part. Say it's Tuesday and you're thinking about going out to eat on Saturday. Swipe forward to Saturday and your Restaurants coin will tell you what you'd have to spend that night, because four more days of your monthly budget will have accrued by then. You haven't spent anything in the meantime, so what you're seeing is the best case, but it answers the question you actually had: can I afford this, and if not, when can I?
Going backward shows you what really happened. Land on a past day and the coins show where you stood that evening, with that day's transactions underneath.
Press and hold the date to jump straight back to today, or tap it to open a picker and go to any day you like.