Every year, the Storybook Ball is a good time for a good cause, raising millions of dollars for the Mass. General Hospital for Children. But this year’s shindig, which takes place Saturday at the Museum of Fine Arts, is the first since the Boston Marathon bombings, making it even more meaningful for organizers. “We already knew that the funds raised this year would go toward pediatric emergency services and trauma programs,” said Kerry Swords, co-chair of the event. “But after the tragic events of last April, we realize more than ever that we have these amazing medical resources in our backyard.” Swords had run the marathon, finishing about an hour before the blasts. She’d just returned to her Beacon Hill abode when her friend, neighbor, and Storybook Ball co-chair Katherine O’Keeffe called in tears to say there’d been an explosion near the finish line. O’Keeffe’s husband had been in his office on Boylston Street when the explosion knocked him out of his chair. “How Mass. General, and all the hospitals, treated the victims efficiently and effectively was amazing,” said O’Keeffe. “We’re lucky to be in a city with so many Level 1 trauma centers. Not one patient who reached the hospital died that day.” As he has since the first Storybook Ball in 2000, party planner extraordinaire Bryan Rafanelli is handling the arrangements Saturday. This year’s theme is the children’s book “Harold and the Purple Crayon,” so guests can expect some whimsical design features, including a purple-crayon-drawn portal leading to dinner. “We’re dreaming big this year and using our purple crayon to support pediatric emergency services at Mass General,” said Swords.