This dataset provides data on the timing of butterfly flight periods for each UK butterfly species across all monitored sites in the UK Butterfly Monitoring Scheme (UKBMS). Annual data from over 2,500 sites are presented relating to the timing (first appearance, last appearance, date of peak abundance and mean flight date) and the duration (total number of days, standard deviation around the mean flight date) of the flight period for all UK butterfly species from 1976 to 2021. In addition, this data is divided each year for eleven multi-voltine species to provide separate phenology data for distinct flight periods associated with first and subsequent generations. Phenology change is a widely used measure of the biological impacts of climate change because of the close relationship between temperature and the timing of biological events. This dataset provides an invaluable tool for assessing the impacts of climate change both spatially and temporally. The UK Butterfly Monitoring Scheme is organized and funded by Butterfly Conservation (BC), the UK Centre for Ecology & Hydrology (UKCEH), the British Trust for Ornithology (BTO), and the Joint Nature Conservation Committee (JNCC). The UKBMS is indebted to all volunteers who contribute data to the scheme. This work was supported by the Natural Environment Research Council award number NE/R016429/1 as part of the UK-SCAPE programme delivering National Capability. Full details about this dataset can be found at https://doi.org/10.5285/0c59eb20-26e3-4066-86f5-418afae18769