I thought for frequent flier tickets. Houston to London in the fall is not exactly high season.
Even the uncolored (white) dates have seats on partner airlines. However, there are only seats from IAH to LHR (London Heathrow) on a non-stop flight on Sept. 6th or 7th, any other date I'd have to connect in either Toronto (YYZ), Washington (IAD), or Newark (EWR). There are no seats on August 28th so the earliest I could leave is the 29th.
If I leave on Monday 8/29, taking my planned itinerary would then have me getting to Berlin on Sept. 16th instead of the 24th as I came up with earlier, giving me plenty of time (17 days vs 9 days) to do Berlin, Prague, Budapest, and whatever I decide to do in Austria before the end of Oktoberfest in Munich on Oct. 3rd - without changing my itinerary at all.
Return flights from Lisbon (LIS) are available any day in October and November via some combination of stops or plane changes in Oporto, Portugal (OPO), Frankfurt (FRA), Washington DC (IAD), Newark (EWR), Munich (MUC), Toronto (YYZ), Chicago (ORD), Boston (BOS), and/or London (LHR).
Of course, that's how it is now, no telling what it'll be like by then.
According to Continental's web page, I can change the date of my return flight whenever I want. If the new date is less than 21 days out, or if I decide to change my departure city for some reason, it's only a $75 change fee.