This is a rather difficult formula with at least one cosinus in it - I once found it out for myself after drawing the Earth, Sun, equator, angles and all - and then found it in a book but now I have forgotten it all. To say it generally - the objects - earth around the sun and a certain point around earths axis - are moving in (almost) circles. If you look at an object moving in a circle from the side (like at the wheel of a bicycle from behind, only the rest of the wheel shouldn't hide the object from our view) it looks like slowing down when getting towards the topmost and the lowest point. Putting the height of the object on the y axis and time on the x axis of a coordinate system you get a sinusoidal curve. And all parameters that are dependent on the position of the object will have a sinusoidal element in their changing too. So days' length change rate also slows down when getting towards its topmost or towards its lowest point.

As translatrix says its a sine wave. Look at the slope. It's changing fastest in March and September and slowest in December and June. Love June.

Nearing the shortest day today in the nothern hemisphere. Thank goodness. It's cold and dark. I hate it!