Funding the December Spike: The Latest Start Month and Monthly Amount That Cover Seasonal Costs Without Borrowing

Seasonal costs are the most predictable large expenses a household faces — the December cluster and the late-summer cluster arrive on the same dates every year — yet they are routinely financed by January debt. This paper simulates the two clusters (medians USD 1150 and USD 420, lognormal) and asks when saving must start, and at what monthly amount, for the fund to cover both clusters in at least 95% of years. Starting in November at USD 200 a month fails in 100.0% of years; starting in September at USD 150 fails in 100.0%; starting in January at USD 100 fails in 79.2%. The cheapest schedule meeting the standard starts in month 2 at USD 290 a month (total paid in USD 3190). The binding constraint is the August cluster, which truncates the saving runway for December. The study is simulated, not measured. SIMULATED, NOT MEASURED.