The article is dedicated to the seasonality of marriages and birth rates in the Old Swedish Colony, as well as the potential influence of a Lutheran pastor on these demographic processes. Hypotheses regarding the connection between marriage dates and the seasonality of firstborn births and the differences in marriage seasonality between the Old Swedish Colony and Josephstal colonies are examined. Metric books and family lists serve as the primary sources. Marriage dates and firstborn birth dates are compared based on data partially reconstructed through family history research methods. Due to the specificity of the sources (lack of surnames, matching names, age information, or its fragmentary nature), it was possible to link personal information regarding marriage records from metric books and family lists for only some couples. Marriage dates and firstborn birth dates were identified for only 63 cases. Spearman’s rank correlation was used to search for correlations, and a correlation between marriage months and the seasonality of firstborn births was found (Rs = 0.32, p = 0.01, n = 63). However, no differences in marriage seasonality between the Old Swedish Colony and Josephstal were detected. Still, differences in the seasonality of firstborn births and subsequent children were observed. In spring, the number of firstborn births was lower compared to later children. Some cases of premarital conception were also identified. Most marriages, both in the Old Swedish Colony and Josephstal, were recorded in September, contradicting the observations of Anton Karlgren (1905), who claimed that most marriages in the Old Swedish Colony occurred in November. Nearly half (48%) of the marriages were double weddings, where multiple couples got married on the same day (2–4 couples). However, the number of such weddings decreased starting from the 1850s. It is worth noting that there was no significant concentration of double weddings in any particular year. The high concentration of double weddings in the late 18th century is explained by compensatory marriage rates after the cholera epidemic. Nevertheless, double weddings in the 1830s‑1850s may have been influenced by the short periods when the pastor visited the colony for wedding ceremonies.The author concludes that pastors had the opportunity to influence the seasonality of firstborn births through the timing of their visits to the colony for marriage ceremonies. However, the most significant impact of pastors on demographic processes was seen in the increase in double weddings, which resulted from the necessity of marrying when the pastor was present in the colony.