This builder is called for every day in the calendar.
If you want to build only few custom day containers, return null for the days you want to leave with default looks
All characteristics like circle border are also applied to the custom day container DayBuilder provides.
(if supplied function returns null, Calendar's function will be called for day).
isSelectable - is between CalendarCarousel.minSelectedDate and CalendarCarousel.maxSelectedDateindex - DOES NOT equal day number! Index of the day built in current visible field
isSelectedDay - if the day is selected
isToday - if the day is similar to DateTime.now()isPrevMonthDay - if the day is from previous month
textStyle - text style that would have been applied by the calendar if it was to build the day.
Example: if the user provided CalendarCarousel.todayTextStyle and isToday is true,
CalendarCarousel.todayTextStyle would be sent into DayBuilder's textStyle. If user didn't
provide it, default CalendarCarousel's textStyle would be sent. Same applies to all text styles like
CalendarCarousel.prevDaysTextStyle, CalendarCarousel.daysTextStyle etc.
isNextMonthDay - if the day is from next month
isThisMonthDay - if the day is from next month
day - day being built.