utils: Align GcalWeekDay enum with ISO 8601

Start the enum from Monday-Sunday (1-7). Adapt code that references the
`is_workday()` utility method, which in turn uses the GcalWeekDay enum
to combine pairs of days by bitwise or-ing.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/822>

src/gcal-enums.h

@@ -51,13 +51,13 @@ typedef enum
 typedef enum
 {
   GCAL_WEEK_DAY_INVALID   = 0,
-  GCAL_WEEK_DAY_SUNDAY    = 1 << 0,
-  GCAL_WEEK_DAY_MONDAY    = 1 << 1,
-  GCAL_WEEK_DAY_TUESDAY   = 1 << 2,
-  GCAL_WEEK_DAY_WEDNESDAY = 1 << 3,
-  GCAL_WEEK_DAY_THURSDAY  = 1 << 4,
-  GCAL_WEEK_DAY_FRIDAY    = 1 << 5,
-  GCAL_WEEK_DAY_SATURDAY  = 1 << 6
+  GCAL_WEEK_DAY_MONDAY    = 1 << 0,
+  GCAL_WEEK_DAY_TUESDAY   = 1 << 1,
+  GCAL_WEEK_DAY_WEDNESDAY = 1 << 2,
+  GCAL_WEEK_DAY_THURSDAY  = 1 << 3,
+  GCAL_WEEK_DAY_FRIDAY    = 1 << 4,
+  GCAL_WEEK_DAY_SATURDAY  = 1 << 5,
+  GCAL_WEEK_DAY_SUNDAY    = 1 << 6
 } GcalWeekDay;
 
 typedef enum

src/gui/common/gcal-date-chooser-day.c

@@ -199,7 +199,7 @@ gcal_date_chooser_day_set_date (GcalDateChooserDay *self,
 
   now = g_date_time_new_now (g_date_time_get_timezone (date));
   today = gcal_date_time_compare_date (date, now) == 0;
-  weekday = g_date_time_get_day_of_week (date) % GCAL_N_WEEKDAYS;
+  weekday = g_date_time_get_day_of_week (date);
 
   if (G_UNLIKELY (today))
     gtk_widget_add_css_class (widget, "today");

src/utils/gcal-utils.c

@@ -549,7 +549,7 @@ struct
 
 /**
  * gcal_util_is_workday:
- * @day: a guint representing the day of a week (0…Sunday, 6…Saturday)
+ * @day: a guint representing the ISO 8601 day of a week (1…Monday, 7…Sunday)
  *
  * Checks whether @day is workday or not based on the Territory part of Locale.
  *
@@ -563,7 +563,7 @@ gcal_util_is_workday (guint day)
   gchar territory[3] = { 0, };
   guint i;
 
-  if (day > GCAL_N_WEEKDAYS - 1)
+  if (day < 1 || day > GCAL_N_WEEKDAYS)
     return FALSE;
 
   no_work_days = GCAL_WEEK_DAY_SATURDAY | GCAL_WEEK_DAY_SUNDAY;