When I worked in London, I always bought odd-period Travelcards. If I was expecting the next couple of months to be 'normal', I would buy a Travelcard valid from Monday of Week 1 to Friday of Week 5 (a 33-day card, which of course varied slightly in price depending on whether it was a 31-day month or not), with the thought that if I didn't travel at the weekend of Week 5 I would save money, and if I did I could get one-day cards and wouldn't lose money. If I knew I was going to be on vacation in the month after the current one, I'd just buy a longer Travelcard to cover me from now until then (often a six or seven week Travelcard).

Maybe that's just my extreme geekdom coming to the fore, but I wondered why anyone would buy a monthly instead of a 33-dayer. Of course, the downside was the somewhat confused booking clerks every time I bought my ticket!