To study the effect of calving month on occurrence probability of atypical lactation curve, a number of 751,099 test day records belonging to 85,641 first parity Holstein cows calved during 1997-2009 in 156 herds in six provinces were used. Logistic generalized linear mixed model using GLIMMIX procedure of SAS software was fit to the data. In the model, fixed effects of province, herd production level, calving year, calving month, age at first calving, sperm type, cow type, interval of first recording from calving, number of total test day records for each cow, determination coefficient of Wilmink fitted to each cow, as well as random effect of sire were included. Dependent variable was defined as a binary (0 for typical and 1 for atypical lactations) in the model. Maximum and minimum probabilities were found for the cows calving in April (0.3435) and December (0.2348), respectively so that cows calving in the former one was expected to have 70.5 % more chance of atypical lactation as compared to the cows calving in the latter one. Based upon the findings it can be concluded that the shape of the lactation curve in Iranian dairy cows is influenced by the seasonal variations.