Highlighting certain dates in mat-calendar

Complementing Fabian's answer, if you have problem seeing the highlighted color and you are using lazy loading components, remember to add the ::ng-deep in front of the class. Like this:

::ng-deep.special-date {
   background-color: red;
}

Cheers!

You can use the the input binding dateClass as describe in the Angular Material documentation here. Note: This requires an Angular Material version of at least 7.1.0

Change your template to this:

<mat-calendar [dateClass]="dateClass()" [selected]="selectedDate" (selectedChange)="onSelect($event)"></mat-calendar>

And in your component add the dateClass function which will generate a function that will return a type of MatCalendarCellCssClasses, which can be as simple as a string representing the CSS class to apply (or an array of class names):

dateClass() {
  return (date: Date): MatCalendarCellCssClasses => {
    if (date.getDate() === 1) {
      return 'special-date';
    } else {
      return;
    }
  };
}

And finally in your styles.css add the classes you want applied:

.special-date {
  background-color: red;
}

Here is a stackblitz that colors the first of each month in red.