Mockito failed

build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.example.circlecisample"
        minSdkVersion 19
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    // Optional -- Mockito framework
    testImplementation 'org.mockito:mockito-core:2.+'
}

Holiday.java

package com.example.circlecisample;

import java.util.Calendar;

public class Holiday {

    private static final int DECEMBER = 12;
    private static final int DAY_OF_XMAS = 25;
    private static final int DAY_OF_XMAS_NIGHT = 24;
    
    public String sayXmas() {
        int month = getCurrentMonth();
        int day = getCurrentDayOfMonth();

        if (month == DECEMBER && (day == DAY_OF_XMAS || day == DAY_OF_XMAS_NIGHT)) {
            return "Merry Xmas";
        } else {
            return "Today is not Xmas";
        }
    }

    protected int getCurrentMonth() {
        Calendar calendar = Calendar.getInstance();
        return calendar.get(Calendar.MONTH) + 1;
    }

    protected int getCurrentDayOfMonth() {
        Calendar calendar = Calendar.getInstance();
        return calendar.get(Calendar.DAY_OF_MONTH);
    }
}

HolidayTest.java

package com.example.circlecisample;

import org.junit.Test;

import static org.mockito.Mockito.*;
import static org.junit.Assert.*;

public class HolidayTest {

    @Test
    public void today_is_xmas_12_25() {
        Holiday holiday = mock(Holiday.class);
        when(holiday.getCurrentMonth()).thenReturn(12);
        when(holiday.getCurrentDayOfMonth()).thenReturn(25);
        assertEquals("Merry Xmas", holiday.sayXmas());
    }
}