Why it matters
Small, unplanned purchases are easy to miss because each one feels too minor to matter. A no-spend day puts a pause in front of them. For one day, the default answer is "not today".
A Yes / No habit keeps it light. You do not need to record every amount, only whether you kept to the day's rule. Many people find the notes are the useful part: what they nearly bought, and when.
This is a common-sense budgeting habit, not financial advice. Essentials such as rent, bills, groceries, transport and medicine always come first. If you are worried about money or debt, look for a free, independent money advice service near you.