Are global variables accessible in Angular 2 html template directly?

No, the scope for code in the template is the component instance. You need to assign the value to a field in the component, or add a getter or method that forwards to the global variable in order to be able to use it from the template.

import { AppSettings } from '../../../app.settings';

...
export class MyComponent {
  get dateFormat() {
    return AppSettings.DateFormat;
  }
}

then you can use it like

<input [(ngModel)]="Holiday" [date-format]="dateFormat"/>

It seems hacky but you can use pipe. It saves you from repeating injection or variable binding for each component.

@Pipe({
    name: 'settings',
})
export class GlobalVariablePipe implements PipeTransform {

    transform(value: any): object {
        return AppSettings;
    }
}

Then, once imported in your module, you can simply use the pipe as follows:

{{(''|settings).DateFormat}}