A calendar rule trades on a date whether or not anything moved. A band trades when something moved, whether or not it is a convenient date.
You set a 60/40 portfolio two years ago. Stocks ran, and it is now 68/32. Nobody decided to take more risk — drift did it quietly. The question is not whether to fix it, but what rule tells you when, because the rule you pick determines how often you trade and how much tax and cost that generates.
— Educational, not financial advice.