The creation of species lists is a quick and relatively cheap method to evaluate the biodiversity value of some sites and as such they are used in various conservation actions. Species lists typically focus on recording rare and endangered species, usually by volunteers who watch birds during their free time, which may lead to an uneven distribution of observations during the week. We examine whether and to what extent a weekend bias exists in the recording by volunteers of rare bird species. We used a database summarizing 1679 records throughout Poland of 154 rare bird species represented by 2433 individual birds. The proportion of individual birds recorded during weekends was significantly higher than expected by chance. The proportion of records of rare birds that were made during weekends varied from 32.4% in July to 54.9% in November and the weekend bias was lowest during the summer holidays (i.e. July and August). Species varied in their weekend bias, however species size and conspicuousness scores did not explain species-specific weekend bias. Species richness as a function of sampled individuals did not differ between weekends and weekdays. We suggest that potential biases caused by increased weekend recording need to be considered when comparing abundances based on lists from different sites or seasons, e.g. in studies on the effect of weather on birds. Our results suggest that the weekend effect is not qualitative but only reflects changes in sampling effort across the week and therefore records from weekends and weekdays are comparable in term of species composition.