I found stats for the US from 1995, from the Monthly statistics report Vol. 45, No. 11, Supplement, from the CDC:
from Table 12:
Total .....................3,899,589
January ....................316,013
February ...................295,094
March .......................328,503
April .........................309,119
May ..........................334,543
June ..........................329,805
July ...........................340,873
August .......................350,737
September .................339,103
October .....................330,012
November ..................310,817
December ..................314,970
I don't see anything that suggests a mating season.