Calculate business days in C#

The base class library has no business-day type. Implement the loop over DateOnly, or call an API that carries the calendars.

Option 1: implement it

static DateOnly AddBusinessDays(DateOnly d, int n, HashSet<DateOnly> holidays)
{
    int step = Math.Sign(n), left = Math.Abs(n);
    while (left > 0)
    {
        d = d.AddDays(step);
        if (d.DayOfWeek is not (DayOfWeek.Saturday or DayOfWeek.Sunday)
            && !holidays.Contains(d)) left--;
    }
    return d;
}

You supply the holidays set per country and keep it current, including substitute days (when 2027-12-25 falls on a Saturday, the UK observes 2027-12-27 and 2027-12-28).

Option 2: Dierum API

using var http = new HttpClient();
http.DefaultRequestHeaders.Add("X-Api-Key", key);

var json = await http.GetFromJsonAsync<JsonElement>(
    "https://dierum.io/v1/add?region=GB&start=2027-12-23&days=3");

json.GetProperty("result").GetString();   // "2027-12-30"

The GB calendar includes the substitute days above, so the result lands on the 30th without local holiday data. Reference covers /v1/count, /v1/due-date, and batch calls for invoice runs.