These docs are for an old release.
Info on upgrading to v4
dayRender
A hook for modifying a day cell’s DOM.
function( date, cell ) { }
This callback lets you modify the day cells elements that are part of each calendar view.
date is the Moment for the given day.
You cannot return a new element. You must only modify the cell (a <td> element) that is provided.
This callback is called each time a cell needs to be freshly rendered.