They explicitly waited to go out until after their entrance exams were completed which were in March. This takes place after March 14th which is White Day in Japan. March is still fairly early in the year and it gets dark early. Plus Mie's parents have a very early curfew for her.It is, they just announced to their parent that they are going out on chapter 101 which the chapter is set on New Year day, so it is safe to assume this is still in Jan or Feb