@{
    ViewBag.Title = "Home Page";

    var times = new List<string>()
    {
        "07:30",
        "08:00",
        "08:30",
        "09:00",
        "09:30",
        "10:00",
        "10:30",
        "11:00",
        "11:30",
        "12:00",
        "12:30"

    };
}

<script type="text/javascript">
    $( "#button" ).click(function() {
        $( "#item" ).toggle();
    });
</script>

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css …" integrity="sha384-GJzZqFGwb1QTTN6wy59ffF1BuGJpLSa9DkKMp0DgiMDm4iYMj70gZWKYbI706tWS" crossorigin="anonymous">

    <h2>KalendarTool</h2>
    <table class="table table-bordered table-striped" id="reservationCalendar" align="center" frame="border" style="width:100%">
        <tr>
            @{

                int currentMonth = DateTime.Now.Month;
                <td style="width:14%"></td>

                for (int month = 0; month < 12; month++)
                {
                    int currentYear = DateTime.Now.Year;
                    DateTime firstDay = new DateTime(DateTime.Now.Year, currentMonth, 1);
                    int daysInCurrentMonth = DateTime.DaysInMonth(firstDay.Year, firstDay.Month);
                    DateTime lastDay = new DateTime(currentYear, currentMonth, daysInCurrentMonth);
                    // Sunday casted to int gives 0 but that will not work for us, we need 7 to be able to calculate number of empty cells correctly
                    int dayOfWeekFirst = ((int)firstDay.DayOfWeek > 0) ? (int)firstDay.DayOfWeek : 7;
                    int dayOfWeekLast = ((int)lastDay.DayOfWeek > 0) ? (int)lastDay.DayOfWeek : 7;

                    for (int i = 1; i <= daysInCurrentMonth; i++)
                    {
                        DateTime renderedDay = new DateTime(firstDay.Year, firstDay.Month, i);
                        String DayDate = renderedDay.ToString("dd.MM.yyyy");

                        if (renderedDay < DateTime.Now )
                        {
                            <td class="alert alert-danger" style="width: 14%">
                                @renderedDay.DayOfWeek, @DayDate <a id="button" href="#">&#9660;</a>
                                <table id="item" style="background: white; width: 100%; padding: 0px; margin: 0px;">
                                    @foreach (string timeOfDay in times)
                                    {
                                        <tr>
                                            <td>
                                                <span>@timeOfDay<input type="button" value="BOOK" style="margin-left: 15px"><input/></span>
                                            </td>
                                        </tr>
                                    }
                                </table>
                            </td>
                        }
                        else
                        {
                            <td class="alert alert-success" style="width:14%">
                                @renderedDay.DayOfWeek, @DayDate
                                <table id="item" style="background: white; width: 100%; padding: 0px; margin: 0px;">
                                    @foreach (string timeOfDay in times)
                                    {
                                        <tr>
                                            <td>
                                                <span>@timeOfDay<input type="button" value="BOOK" style="margin-left: 15px"><input /></span>
                                            </td>
                                        </tr>
                                    }
                                </table>
                            </td>

                        }

                        if (renderedDay.DayOfWeek == DayOfWeek.Sunday)
                        {
                            @:</tr>
                            @:</table>
                            @:<table class="table table-bordered table-striped" id="reservationCalendar" align="center" frame="border" style="width:100%">
                            @:<tr>
                        }
                    }
                    currentMonth = currentMonth + 1;
                }
            }
        </tr>
    </table>