As of the modifications about the regional and optional holidays in Netherlands (see: #571 and #569), it's now possible to pass arguments to the calendar constructor. As a consequence, integrators could pass "options" for a given calendar.

It's a brand new API opening up, and it may enable to fix some calendars. For example, if an administration is following a certain calendar, but in some cases wants to disable a holiday, occasionnaly (i.e., not on a regular basis).

Use with care, though.