If you're hoping to avoid unpleasant surprises (anyone else part of the "no white after Labor Day" boycott?), Clue lets you monitor your cycle and know when your period is on its way. It also tracks symptoms and fertility window, all in a simple and streamlined design that keeps it easy to navigate. Plus, compared to similar apps, the name doesn't immediately scream "PERIOD!" so no need to hide it in a vaguely named folder.
Available for iOS and Android