this is one of the best novel type game that i played good story beautiful graphics and a fast developement so just saying that i like what you r doing hope you will continue :)
If you are still wondering (wich i dont think you are) you need to grab the swimsuit at a precise day you probably go the right swimsuit for the other girl at the right day so just look into the notes and grab it the right day :)
there is not only one way for each girl dont worry i finished the game in 0.6.1F and got four special scene one for each girl in only one playthrough. But i think if you want to really enjoy the game you kinda have to do everithing thats how i did it anyway. :)