calendar -- a gtk:calendar widget
year -- an unsigned integer with the year for which details are needed
month -- an unsigned integer with the month for which details are needed
day -- an unsigned integer with the day of month for which details are needed
result -- a string with Pango markup with details for the specified day, or nil

This kind of functions provide Pango markup with detail information for the specified day. Examples for such details are holidays or appointments. The function returns nil when no information is available.