Sonny, in the book, seems to mention that it is three days between Don Vito getting shot and Michael proposing that he would kill Solozzo and McCluskey. Olivant is right, there is nothing that says a specific date, but it is clearly around the X-Mas season.
Xmas, because the novel and the film refer to or show Xmas shopping. Plus the novel says that Michael intended to marry Kay "the week of Christmas."
