In 1980, George Brett hit .390, flirting with the magical .400 line all season long. He was hitting .406 on August 30th and was at .400 as late as September 19, game number 148 on the season. In the last forty years, we've really only seen a handful of players get this close to .400.
What drove Brett's quest that season was an absolutely insane July. Brett played regularly through June 10, before hitting the DL. To that point, he was having a MVP-level season, but not necessarily an all-time great campaign, hitting .337/.407/.609. Ho, hum.
On July 10th Brett returned, going 2-4 in a 3-2 Royals win (Splitt picked up the win). The next day he went 3-5 as Jack Morris expertly pitched to the score in a 7-3 Royals win. And that was pretty much the entire month. Brett hit .586/.636/.862 in his first seven games back, raising his batting average to .374. A 2-3 game on the final day of the month pushed him all the way up to .390.
Overall his July line was .494/.541/.812 in 98 PAs. Crazy.
In August he hit .430 and got over .400 on the season. Too bad he only hit .324 in September.
So really, we should just pause and reflect on that month. One of the greatest players of all time, in one of his best seasons, in the middle of an absolutely insane hot streak. Your homework assignment is to find other singularly great Brett months.