Widget disappears from my Samsung Galaxy after every update - day-by-day-calendar/feedback GitHub Wiki
That's the problem of Android on Samsung devices. Other apps (not only Day by Day Calendar) have to deal with this problem as well. The bug is described on Samsung's official forum and keeps being unfixed since April, 2014. Although the same might happen to non-Samsung devices. Most probably it's caused by launchers. Standard Samsung launcher is TouchWiz.