add some comments & fix test case name

internal/habits/habits.go

@@ -73,6 +73,7 @@ func (s source) readCells(rangeName string) ([][]interface{}, error) {
 // toCards returns a slice of trello cards from the given habits which haven't been marked today
 func toCards(habits map[string]habit, label string) (cards []trello.Card, err error) {
 	for name, habit := range habits {
+		// proper cell names are 11/12 characters long on single/double digit days of month
 		if len(habit.CellName) != 11 && len(habit.CellName) != 12 {
 			return nil, fmt.Errorf("illegal habit cell name: '%s' %w", habit.CellName, err)
 		}
@@ -81,6 +82,7 @@ func toCards(habits map[string]habit, label string) (cards []trello.Card, err er
 			continue
 		}
 
+		// include the day of month in card title to force overwrite in the beginning of the next day
 		title := fmt.Sprintf("%v (%s)", name, habit.CellName[10:])
 		c, err := trello.NewCard(title, label, habit.CellName, nil)
 		if err != nil {

internal/habits/habits_test.go

@@ -49,7 +49,7 @@ func TestToCards(t *testing.T) {
 			err:      errors.New(""),
 		},
 		{
-			name:  "some marked some unhabits",
+			name:  "some marked some unmarked habits",
 			label: str,
 			habits: map[string]habit{
 				"a": {cellName, "✔"},