This article discusses Cigugur indigenous communities’ custom in carrying out calculation practices to determine the best time to start house construction activities. The realist ethnography approach was used in this study through exploratory research. The ethnomathematics design adopted four elements: generic questions, initial answers, critical constructs, and specific activities. Ethnomodelling was used to translate cultural mathematics ideas into academic mathematics through ethical, emic, and dialogical approaches. Based on ethnomathematics, the community practices the dimensions of necessary universal mathematical activities, such as counting, placing, and explaining. Based on ethnomodeling, mathematical ideas and practices carried out are relevant to the concepts of enumeration, integer operations, sets, relations, congruence, and modulo. The study results recommend the importance of ethnomodelling as a methodological approach to ethnomathematics to create and develop didactical situations, especially in developing a new lesson.