Seasonal swarming by bats in underground sites in late summer and early autumn is increasingly understood to play a vital role in their life-cycle, relating both to nuptial activity and to the use of swarming sites as hibernacula. The common pipistrelle (Pipistrellus pipistrellus) is a largely commensal species in which seasonal swarming often occurs in built structures, which have been hypothesised to serve as mass hibernacula during the coldest part of the winter. A number of detailed studies of P. pipistrellus swarming sites have recorded year-round patterns of activity, as well as the demographic make-up of the visiting bats, but there has been relatively little study of overnight patterns of activity, or how swarming interacts with weather conditions at a variety of scales. This study uses auditory monitoring data, recorded continuously over a 27 month period in a P. pipistrellus swarming site within a built structure in northern England, to derive a detailed phenology of bat activity within the site, and uses generalised additive modelling to explain daily variation in activity in terms of seasonal trends modified by fluctuating weather conditions. Results suggest that cool conditions delay the onset of swarming in spring and late summer, and can suppress swarming behaviour even at the height of the swarming period, leading to a pronounced pulse of activity when temperatures rise, and the extension of activity into mid-Autumn. Roosting also occurred at the study site during the swarming period, and was associated with significant flying during the day in warm temperatures, which may reflect a search for cooler roosting locations. Overnight activity occurred throughout the hibernation period, but little evidence emerged for an influx of bats to a mass hibernaculum at the start of the winter.