target_time = {hour=hour, min=minute, sec=second}

function get_total_day_seconds (time_table)
    return (((time_table.hour * 60) + (time_table.min)) * 60) + time_table.sec
end

SECONDS_PER_DAY = get_total_day_seconds({hour=24, min=0, sec=0})

function get_sleep_duration_sec ()
    current_time = os.time()
    current_time_table = os.date("*t", current_time)
    current_day_seconds = get_total_day_seconds(current_time_table)
    target_day_seconds = get_total_day_seconds(target_time)
    difference = target_day_seconds - current_day_seconds
    if difference > 0 then
        return difference
    else
        return SECONDS_PER_DAY + difference
    end
end

function get_weekday ()
    return os.date("%a", current_time)
end

function sleep (sec)
    return os.execute("sleep " .. tonumber(sec))
end

while true do
    sleep_duration = get_sleep_duration_sec()
    print("sleep for " .. sleep_duration)

    if not sleep(sleep_duration) then
        print("sleep exited abnormally - break")
        break
    end

    if get_weekday() == weekday then
        print("send message " .. message)
        sender:send({type = "message", channel = channel, message = message})
    end
end