This publication presents a collection of materials on the chrononymy of the Kostroma Region from the 20th and 21st centuries. The field data were collected by the Toponymic Expedition of Ural University between 2003 and 2024. The corpus of chrononyms is large, containing over 5,100 entries. Due to its size, the calendar materials will be published in two parts, based on the traditional seasonal division of the calendar year: spring-summer and autumn-winter cycles. Each part is divided into two sections: fixed dates and periods are presented first, followed by movable ones. For the spring-summer cycle, fixed holidays range from March 4 to August 31, while movable holidays cover the period from the start of Lent to the end of summer. The information is presented as follows: 1) the calendar date (in the Gregorian calendar), 2) the canonical name of the church holiday or widely recognized name of the day or period, and 3) all local name variants for the day/period (listed alphabetically, along with the districts of the Kostroma region where they were recorded). Examples of the chrononym’s use in popular speech are also provided, typically including calendar-related signs, proverbs, agricultural recommendations, or descriptions of festive rites. The authors aim to reconstruct the regional system of calendar names, while also providing ethnocultural context. Many of the materials presented are newly collected and have not previously been published. Some of the Kostroma chrononymic data collected by the Toponymic Expedition were included in the all-Russian chrononym collection Russian Folk Calendar (Moscow, 2015), but the fieldwork conducted over the past decade has significantly expanded both the geographical scope and the volume of the data.