Abstract Rationale Amid the ongoing coronavirus disease 2019 (COVID-19) pandemic in which many countries have adopted physical distancing measures, tiered restrictions, and episodic “lockdowns,” the impact of potentially increased social mixing during festive holidays on the age distribution of new COVID-19 cases remains unclear. Objective We aimed to gain insights into possible changes in the age distribution of COVID-19 cases in the UK after temporarily increased intergenerational interactions in late December 2020. Method We modelled changes in time use and social mixing based on age-stratified contact rates using historical nationally-representative surveys and up-to-date Google mobility data from four weeks before and after the festive period. We explored changes in the effective reproduction number and the age distribution of cases, in four scenarios: (1) “normal”: time use and contact patterns as observed historically, (2) “pre-lockdown”: patterns as seen before the lockdown in November 2020, (3) “lockdown”: patterns restricted as in November 2020, and (4) “festive break”: similar to 3 but with social visits over the holiday period as in 1. Results Across ages, the estimated Reff decreases during the festive break in scenarios 1-3 and returns to pre-holiday levels in scenarios 2-3, while remaining relatively stable in scenario 4. Relative incidence is likely to decrease in children aged 0-15 but increase in other ages. Changes in age distribution were large during the holidays, and are likely to start before the holidays for individuals aged 16-24 years in scenarios 1-3. Conclusions Our modelling findings suggest that increased contacts during the festive period may shift the age distribution of COVID-19 cases from children towards adults. Given that COVID-19-related hospitalisations and deaths rise by age, more intergenerational mixing risks an increased burden in the period following the holidays. Highlights Home visits are associated with increased intergenerational mixing. The effective reproduction number is likely to remain stable or even reduce slightly due to a reduction in contacts at work and school. Relative incidence is likely to become lower in children, but higher in the older (more vulnerable) age groups around the holiday period, which could lead to increased health care burden.