local function get_day_no(day)
    -- Throws an error if day is invalid.
    return switch day:lower() do
        case "monday"    -> 1
        case "tuesday"   -> 2
        case "wednesday" -> 3
        case "thursday"  -> 4
        case "friday"    -> 5
        case "saturday"  -> 6
        case "sunday"    -> 7
        default          -> error($"'{day}' is not a valid day in English.")
    end
end

for {"thursday", "jeudi"} as day do
    local ok, res = pcall(get_day_no, day)
    if ok then
        print($"'{day}' is day number {res}")
    else
        print(res)
    end
end