C# web form - Looking to get the date of the first monday of the month after the selected date from the calendar

Question

I am currently creating a web form in Visual C#, the app is to be a finance calculator.

Most of the code below is extraneous and was only included to show the whole page, as detailed in the title, I am looking to be able to select a date from the calendar on the web form and from there take the date of the first Monday and display it in the label. I can currently select a date and have that displayed.

namespace FinanceCalc
{
    public partial class About : Page
    {
        decimal NumPmt = 0;

        protected void Page_Load(object sender, EventArgs e)
        {
        }

        protected void Price_TextChanged(object sender, EventArgs e)
        {
            if (String.IsNullOrEmpty(Price.Text))
            {
                Response.Write("Please enter the Vehicle Price");
            }
        }

        protected void DepositCalc_Click(object sender, EventArgs e)
        {
            if (String.IsNullOrEmpty(Price.Text))
            {
                Response.Write("Please enter the Vehicle Price");
            }
            else
            {
                decimal price = Convert.ToDecimal(Price.Text);
                decimal depositamount = (price / 100 * 15);

                Deposit.Text = depositamount.ToString();
            }
        }

        protected void FinanceCalc_Click(object sender, EventArgs e)
        {
            // This works out if all required boxes have been populated and displays a warning message if they have not been.
            if (String.IsNullOrEmpty(Price.Text))
            {
                Response.Write("Please enter the Vehicle Price.");
            }

            if (String.IsNullOrEmpty(Deposit.Text))
            {
                Response.Write("Please calculate the deposit.");
            }

            if (String.IsNullOrEmpty(Length.Text))
            {
                Response.Write("Please enter the finance length.");
            }

            if (Convert.ToInt32(Length.Text) < 1)
            {
                Response.Write("Please choose between 1, 2 and 3 years.");
            }

            if (Convert.ToInt32(Length.Text) > 3)
            {
                Response.Write("Please choose between 1, 2 and 3 years.");
            }
            else
            {
                // This works out payment amounts
                NumPmt = 12 * int.Parse(Length.Text);
                decimal price = Convert.ToDecimal(Price.Text);
                decimal deposit = Convert.ToDecimal(Deposit.Text);
                decimal MonthlyPayments = (price - deposit) / NumPmt;

                // This populates the Finance Info text box with the information
                Results.Text = "Deposit Amount - £" + Deposit.Text + Environment.NewLine + "Arrangement Fee - £88" + Environment.NewLine + "Completion Fee - £20" + Environment.NewLine + "Number of Payments: " + NumPmt
                + Environment.NewLine + "Delivery Date: " + DeliveryDate.SelectedDate + Environment.NewLine + "First Payment Date: " + Environment.NewLine + "First Payment Amount: £" + MonthlyPayments + " plus your £88 Arrangement Fee."
                + Environment.NewLine + "Monthly Payments: £" + MonthlyPayments + " for " + (12 * int.Parse(Length.Text) - 2) + " months." + Environment.NewLine + "Final Payment Amount: £" + MonthlyPayments + " plus your £20 Completion Fee";
            }
        }

        protected void Reset_Click(object sender, EventArgs e)
        {
            //This resets all of the information boxes, allowing the user to start again
            Price.Text = "";
            Deposit.Text = "";
            Results.Text = "";
        }

        protected void Length_TextChanged(object sender, EventArgs e)
        {
            // This works out how many monthly payments will be made. 
            NumPmt = 12 * int.Parse(Length.Text);
        }
    }
}    

Answer 1

This should do the job:

var selectedDate = new DateTime(2018, 5, 2);
var firstMonday = Enumerable.Range(0, 2)
    .SelectMany(monthOffset => Enumerable.Range(1, 7)
        .Select(day => new DateTime(date.Year, date.Month + monthOffset, day)))
    .Where(x => x.DayOfWeek == DayOfWeek.Monday)
    .First(x => x > selectedDate);