Seasonality in milk yield and SCC in the US can be partially attributed to a seasonal calving pattern, but detailed information of the magnitude is limited. The aim of this study was to describe the seasonality with or without adjustment for calving pattern in milk yield/cow/d (MILK), and SCC of dairy herds across the US in 2016. The regions analyzed (herds) were Southeast (SE, 183), Northeast (NE, 2,332), and Midwest (MW, 2,240). We excluded farms that were expanding or shrinking, lactations with less than 5 test-day observations, and herds with fewer than 15 cows per lactation group (LACT). Sine and cosine functions were used to describe seasonality per herd per LACT to find the annual low (minimum) and peak (maximum) of MILK and SCC. Annual low-to-peak ratios were measures of maximum seasonality, and were not adjusted (LP) or adjusted (LPADJ) for cows’ days in milk to correct for seasonal calving. Traditional summer-to-winter (SWr) ratios of unadjusted observations were used for comparison. The median seasonality (SWr, LP, LPADJ) for MILK was 0.88, 0.80, 0.83 (SE); 0.99, 0.87, 0.89 (NE); and 0.98, 0.87, 0.88 (MW). For SCC, the values were 1.15, 0.46, 0.47 (SE); 1.14, 0.46, 0.47 (NE); and 1.17, 0.47, 0.47 (MW). Adjusting for calving pattern resulted in the greatest decrease in seasonality for MILK in the SE, by ((1–0.80)-(1–0.83))/(1–0.80) = 15%, but not for SCC. Overall, LPADJ reduced seasonality for at least one of the LACT in 87% of the SE herds for MILK (mean decrease of 29%), and in 76% of the SE herds for SCC (decrease of 8%). Overall, LACT 1 cows were less seasonal than older cows. For MILK, the median dates (month/day) of the low and peak for LP were 09/15, 03/21 (SE); 09/16, 05/05 (NE); 09/15, 05/06 (MW). For SCC, LP low and peak dates were 04/10, 09/15 (SE); 04/19, 08/31 (NE); 04/18, 08/24 (MW). In conclusion, LP ratios were more accurate measures of seasonality than SWr. LPADJ models were able to adjust for the effects of calving pattern on seasonality.