Why Is April 1 “Census Day”?
As Anderson (1988:44) notes, “census takers always knew that the count could be affected by the month of the year it was taken.” Early American censuses had to balance the difficulty of making personal contact with residents (slowly, with transportation by foot or horseback) with the prospects of duplicate counting or omissions that would follow from allowing the enumeration to run too long. The first four decennial censuses all used an early August date as the reference, since the summer and fall months were judged to be the best time to find people in what was still an agricultural society. For 1830, “Congress also moved the date of the census ahead two months, to June 1 instead of August  as in 1820, on President Adams’s suggestion that this would permit a longer stretch of good weather for the house-to-house enumeration” (Cohen, 2000:121). June 1 remained the census date for the 1840 through 1900 censuses.
As the census became more routinized and professional, officials worked to shorten the time of the count and shift the census date earlier in the year. The increasing urbanization of the United States prompted the shift from June 1 to April 15 in the 1910 census; June was “unsatisfactory…because some city dwellers were already out of town for summer vacations, and farmers did not remember enough about the previous year’s crop for the agricultural census” (Anderson, 1988:44). The Census Bureau tried moving Census Day back further still in the 1920 census, to January 1, “at the request of the Department of Agriculture, and also because it was contended that more people would be found at their usual place of abode in January than in April” (Steuart, 1921:571). However, this change “ran into trouble because the winter weather impeded the enumeration and rural leaders complained that many people were working in the city community and hence were not properly counted” (Anderson, 1988:45).
After the 1920 experience, census officials were “convinced…that a more nearly perfect and a more rapid count of the people can be made in April than in January.” However—presaging the continuing problem of counting seasonal residents—they acknowledged that this represented a tradeoff (Steuart, 1921:572):
It is true that during April and June, when the enumeration has heretofore been in progress, large numbers have been at summer resorts. But at [the January 1920] enumeration it was found that surprisingly high numbers were at winter resorts. Thousands who have their usual places of residence in the northern states spend the winter months in California, Florida, and other southern states. Some of them live in the south several months of each year, and it was difficult to determine their usual places of abode. In this respect the change complicated the work; certainly it did not simplify it.
Hence, the 1930 census set April 1 as Census Day, and that date has since been written into Title 13 of the U.S. Code for subsequent censuses. One exception in the 2000 and other recent censuses is the enumeration of remote villages in Alaska, which are rendered unreachable by weather conditions in March and April. In 1930, the count there only began in October; recent censuses have tallied those areas in January or February.
The earliest decennial censuses were conducted by marshals on horseback; though the federal agency charged with conducting the census varied,