Background: The festive season is associated with weight gain that often persists into the new year, but the specific purchasing behaviours driving this are not well quantified. Christmas is also thought to be a period of disproportionate environmental impact, though the contribution from food purchasing has not been explored. The objective of this study was to use retail transaction data from a major UK supermarket to examine seasonal patterns in purchases of less healthy food and drink products (High in Fat, Sugar and/or Salt, HFSS), and the greenhouse gas emissions (GHGE) from sales, and to quantify the extent to which purchasing behaviours during the festive period differ from the rest of the year.Methods: Daily food and non-alcoholic beverage transaction data for 2022 for loyalty card holders living in Yorkshire and Humber were obtained from a major UK retailer. Products (>27,000) were classified as HFSS or non-HFSS using the UK Nutrient Profile Model and linked to GHGE estimates per kg. Changes in HFSS sales and GHGE were examined between the festive period (1-31 December 2022) and the baseline period (1 Jan-30 Nov 2022) using four metrics, expressed as the daily average over the two periods: HFSS sales (kg), percentage of all sales by weight that were HFSS (%HFSS), total GHGE from sales (kg-CO2-eq) and GHGE per kg (kg-CO2-eq·kg-1). Results: Marked increases in total HFSS sales and GHGE from sales were observed in the festive period (+25% and +15.3% respectively). The additional festive GHGE footprint was equivalent to 3,000 return flights from London to the North Pole. Consumers purchased proportionally more HFSS foods (+10.1%) and more foods with higher GHGE per kg (+6.6%). These changes were driven by shifts towards products that were both less healthy and more environmentally costly, particularly beef, chocolate and lamb.Conclusion: Christmas is traditionally associated with generosity, celebration and plentiful food. Our study shows that this festive cheer comes at a cost to health and the environment. With a little more thought about what food we buy, the festive period can still be enjoyed while also benefiting population and planetary health.