BTBS article on April W/L records: A recent study conducted of Phillies' monthly winning percentages (dating from 2000 to 2011) indicates a minor correlation between April success (or lack thereof) and end-of-season winning percentage.
Instead, the study indicates performances in the months May and June are more telling than April.
To begin the study, I found Phillies' month-by-month records from 2000 to present on Baseball Reference and placed them in Excel. I then ran a multiple regression analysis with the Y variable equaling season winning percentage and the X variables equaling the various monthly winning percentages. I did not include the instances where games were played in March or October:
* April: .127
* May: .244
* June: .219
* July: .143
* August: .175
* September: .178
The higher the number, the more closely that month is linked to the end-of-season winning percentage.
(Not exactly peer-reviewed, but interesting nonetheless, considering the current Royals April Disaster.)