The article explores the potential of large language models in thematic analysis of historical texts, exemplified by the 1849 diary of Vologda gymnasium student Kirill Antonovich Berezkin. This rich source illuminates the everyday life, worldview, and social interactions of a young individual in mid-19th century provincial Russia. The diary offers a multifaceted narrative, capturing cultural events, political contexts, and personal introspections. By meticulously analyzing this text, researchers can reconstruct not just an individual's experiences, but also gain profound insights into the social, cultural, and educational landscape of the era. Employing the Gemini 1.5 Pro model, renowned for processing extensive textual data, the study conducted a comprehensive analysis. The research methodology involved examining the diary both holistically and through monthly segmentation, enabling the identification of nuanced content aspects. The novelty of the approach lies in applying modern large language models to a Russian historical document. The results demonstrated the model's remarkable capability to identify key themes, successfully isolating eight major thematic areas that reflect the gymnasium student's life. Utilizing parallel prompting with a monthly text breakdown revealed specific themes and subtleties that a comprehensive review might have overlooked. The study ultimately validates the effectiveness of large language models in historical source analysis, presenting promising opportunities for automating topic modeling and uncovering hidden patterns in extensive textual datasets. However, the inherently stochastic nature of these models necessitates multiple analyses, careful result interpretation, and critical comparison with traditional historical research methodologies.