The calendar is shown on top of a picture of your choice- just click the camera icon in the lower left corner to select a picture to use as the calendar background.
Once the picture is in place, you can change how the calendar looks on top of it.
To make the calendar more transparent, stroke the screen downward and then hold your finger till it's the level you like. To make it more opaque, stroke upward and hold.
One accent color is used in the calendar- stroke the screen left or right and hold to cycle the color.
Double-tap the screen to change the color scheme- some look better on light photos, and others look better on dark photos.
On the bottom of the screen is a toolbar with the following buttons:
The camera, to pick the background picture.
Left and right arrows to see the previous or next month.
The home button will take you back to the current month.
The (i) Information button takes you to the developer website.
One more thing- if you turn the iPhone sideways, you can see the whole year at once instead of a single month. The calendar shows a whole month in each row.