Background/objective Medication errors are one of the main causes for adverse events in hospitals. This study aims at describing reported in-hospital medication errors and their relationship with night shifts and weekdays versus weekends and/or holidays over a 5.5-year period in a large university hospital in the French speaking part of Switzerland. Methods A retrospective analysis was conducted on data from the hospital adverse and critical event database (prescribing and administering) covering January 1st 2014-June 18th 2019. In-house medication errors were classified as "prescription errors, side effects, contradictions," "wrong patient," "omission of delivery," "calculation problem including wrong dosage or dilution," "problem in flow, duration and time of administration," "problem in technique, route and form of administration" and "blood-related errors." Results Overall, 1626 reported in-hospital medication error events from 2014 to mid-2019 were included in the analysis. The most frequent medication error was "calculation problem" (24.2%), while "wrong patient" showed an increasing trend over the study period (4.8-14.8%). Medication errors occurring in weekends and/or holidays accounted for over one fifth of all errors, while medication errors occurring on night shifts varied between 23.9% and 29.6%. On a daily pattern, "calculation problem" and "wrong patient" peaked at 8 h, "prescription errors" at 15 h and "flow, duration and time" at 18 h. On multivariable analysis, "prescription errors" and "technique, route and form of administration" occurred more frequently during weekdays [odds ratio (95% confidence interval) 2.14 (1.47-3.12), p < 0.001, and 1.51 (1.02-2.25), p = 0.038, respectively], while "omission of delivery" and "flow, duration and time" occurred less frequently during weekdays [0.56 (0.34-0.93), p = 0.026, and 0.69 (0.53-0.91), p = 0.008, respectively]. Conclusion In a Swiss university hospital, preventable medication error events are frequent and their types have differing time occurrences, thereby suggesting the necessity for tailoring preventive measures to specific time patterns.