Background/Objectives: Hip arthroplasty is highly effective but carries a measurable burden of early postoperative adverse events. Knowing not only how often these events occur but when they appear is relevant to patient safety and postdischarge surveillance. The aim of this study was to describe the frequency and timing of the main adverse events within 90 days of total or partial hip arthroplasty and to compare onset timing between prosthesis types. Methods: This was a retrospective single-center observational cohort study with a longitudinal survival component. A total of 212 consecutive patients who underwent total or partial hip arthroplasty during 2023 at a tertiary university hospital were included. Five adverse events (postoperative fever, surgical-wound infection, prosthesis infection, reoperation, dislocation) were recorded for 90 days from surgery. Time-to-onset was analyzed with the Kaplan–Meier method and compared between total and partial prostheses with the log-rank test (SPSS v25; two-sided p < 0.05). Results: A total of 167 patients (78.8%) received a total and 45 (21.2%) a partial prosthesis; approximately 29% experienced at least one adverse event. Postoperative fever was the most frequent (58; 27.4%) and earliest event (median onset 2 days, 95% CI 1.5–2.5), followed by surgical-wound infection (17; 8.0%), reoperation (11; 5.2%; median 25 days), prosthesis infection (8; 3.8%; latest onset day 77) and dislocation (5; 2.4%; median 15 days). Onset timing differed between prosthesis types only for fever (median 1 vs. 3 days; p = 0.016), an exploratory difference given the different clinical profiles of the two groups. Conclusions: Within 90 days of hip arthroplasty, fever is the most frequent and earliest adverse event, whereas wound and prosthesis infections, reoperation and dislocation may present well beyond hospital discharge. These exploratory findings suggest that structured postdischarge follow-up may warrant further evaluation.