Health issues mean I can potentially wake up feeling dog-rough any day. Therefore there is no point me booking ahead for say, next Wednesday, in case I end up too tired to attend. I therefore draw up a hotlist of candidates who will likely be available next Wednesday, note their phone numbers and if I'm up for it on awakening I get on the phone and get something booked for later that day, usually just a couple of hours later.
Often I bomb out.... None available or not answering. I then curse my fucked up vestibular system and have a wank instead.