How to calculate business days in C#

There is no BCL business-day type; here is the honest landscape.

1. Roll your own over DateOnly

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;
}

The code is the easy 10%. The holidays set, per country, with substitute days, updated when governments change rules, is the hard 90%.

2. One HTTP call

var res = await http.GetFromJsonAsync<JsonElement>(
    "https://<host>/v1/add?region=GB&start=2027-12-23&days=3");
// skips Christmas, Boxing Day AND both substitute days -> 2027-12-30