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Simple Summary: The days open was calculated as the difference between the dates of successful insemination and previous calving, which is a critical trait for the dairy industry. However, factors affecting days open were not well documented, and clarifying the mechanism of days open will contribute to the high-quality development of the dairy industry. Therefore, the objectives of this study are to evaluate the difference in days open between parity or calving season groups and assess associations of days open with wavenumbers ranging within the mid-infrared region. We found that the days open in calving season groups were significantly different, and dairy cows having a calving event in autumn showed the shortest number of days open. Weak associations were detected between days open and wavenumbers spanning the mid-infrared region. Based on the findings of the study, the reproduction management of dairy herds can be dynamically optimized according to the calving seasons, and the relationship between wavenumbers and days open needs to be further confirmed to service the dairy industry better. Milk spectral data on 2118 cows from nine herds located in northern China were used to access the association of days open (DO). Meanwhile, the parity and calving season of dairy cows were also studied to characterize the difference in DO between groups of these two cow-level factors. The result of the linear mixed-effects model revealed that no significant differences were observed between the parity groups. However, a significant difference in DO exists between calving season groups. The interaction between parity and calving season presented that primiparous cows always exhibit lower DO among all calving season groups, and the variation in DO among parity groups was especially clearer in winter. Survival analysis revealed that the difference in DO between calving season groups might be caused by the different P/AI at the first TAI. In addition, the summer group had a higher chance of conception in the subsequent services than other groups, implying that the micro-environment featured by season played a critical role in P/AI. A weak linkage between DO and wavenumbers ranging in the mid-infrared region was detected. In summary, our study revealed that the calving season of dairy cows can be used to optimize the reproduction management. The potential application of mid-infrared spectroscopy in dairy cows needs to be further developed. [ABSTRACT FROM AUTHOR]