看起来您的主要问题是initState()中的这一行：

DateTime d = DateTime.utc(DateTime.now().year, DateTime.now().month, DateTime.now().day, 12);

eventLoader在午夜（00：00：00.000）被传递DateTime s。当你把“事件A”添加到你的事件Map时，你给予它的键是DateTime，表示今天中午，因为你给它的小时参数是12。在事件加载器中，你检查你的事件Map中是否有一个值的键是今天午夜。因为没有，它认为今天没有事件。
您还缺少TableCalendar的firstDay和lastDay参数，这两个参数都是必需的。
你提到的错误似乎来自你发布的代码片段之外的代码。你唯一使用的“year”getter是DateTime.now().year，这是完全有效的。
下面是一个应用了这两个修正的日历小部件的示例。我还向代码添加了其他一些小的改进，并添加了解释这些改进的注解。下面是一个显示生成的日历的图像链接，它正确地显示了添加到今天日期的事件：
The resulting calendar running in an iOS simulator

class Calendar extends StatefulWidget {
  const Calendar({Key? key}) : super(key: key);

  @override
  State<Calendar> createState() => _CalendarState();
}

class _CalendarState extends State<Calendar> {
  final Map<DateTime, List> _events = {};

  DateTime _focusedDay = DateTime.now();

  // Instead of initializing in initState, just add the "late" modifier.
  late DateTime _selectedDate = _focusedDay;

  @override
  void initState() {
    // This is where your problem is. The event loader is passed DateTimes with
    // the time component set to zero (midnight). d is set to noon. Just delete
    // the argument of "12" for hours.
    DateTime d = DateTime.utc(
      DateTime.now().year,
      DateTime.now().month,
      DateTime.now().day,
    );
    // Just add "Event A" to the list on this line
    _events[d] = ["Event A"];
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        TableCalendar(
          // Make sure you add the required arguments "firstDay" and "lastDay"
          firstDay: DateTime(1970),
          lastDay: DateTime.now(),
          focusedDay: _focusedDay,
          onDaySelected: (selectedDay, focusedDay) {
            if (!isSameDay(_selectedDate, selectedDay)) {
              setState(() {
                _selectedDate = selectedDay;
                _focusedDay = focusedDay;
              });
            }
          },
          eventLoader: (day) {
            // Use a null aware operator "??" to make this line simpler. If
            // _events[day] is null, return the empty list instead.
            return _events[day] ?? [];
          },
        )
      ],
    );
  }
}