["Length of hospital stay (LOS) is a large driver of cost after primary total joint arthroplasty (TJA). Strategies to decrease LOS may help reduce the economic burden of TJA. This study's aim was to investigate the effect of day of the week of surgery on mean LOS and total charges following primary total knee arthroplasty (TKA) and total hip arthroplasty (THA).", 'An administrative clinical database at a large US health care system was reviewed for all primary THA and TKA admissions performed between 2010 and 2012 (n\xa0= 15,237). Of these, 14,800 cases met our inclusion criteria and were analyzed. Furthermore, the cohort was divided into early (Monday/Tuesday) and late week (Thursday/Friday) surgeries, excluding Wednesday surgeries (n\xa0= 2835). Univariate and multiple regression analyses examined the effect of each variable on LOS.', 'Mean LOS for THA and TKA on Monday was 3.54 and 3.35 days and increased to 4.12 and 3.66 days on Friday (P < .0001), respectively. Late vs early week admissions had 0.358 (95% confidence interval: 0.29-0.425, P < .001) additional hospital days. Increased age (0.003 days per unit increase in age, P\xa0= .02) and severity of illness score (0.781 days per level increase, P < .001) were associated with increased LOS. Late week surgery had a greater effect on LOS for TKA than for THA. TKAs were associated with higher charges for late week surgery vs early week surgery (P < .001).'] Does day of Surgery affect Length of Stay and Charges in Primary Total Hip and Knee Arthroplasty?