From the March 16, 1945 Kansas City Call: "The Kansas City Monarchs will open their spring training grind at Houston, Texas, Tuesday, March 20. ... Of course the prize freshman in camp will be Jackie Robinson, who most likely will not report until the first week of April. Robinson must straighten out his chores at Sam Houston college, Austin, Texas where he is coach."
Check the image for the full story, as well as the Negro American League schedule for the first half of the '45 season. (The schedule was split into two halves, with the winners of each half meeting in a post-season championship series. Or if one team won both halves, they were declared champions.) May 6 is the first game of the regular season, but the first spring training game is slated for April 1.