With the advent of dual specs, it's become even more possible to run a very tight ship in terms of player numbers. Once you solve the loot problem -- giving offspec loot on the same priority as mainspec isn't logical, but not giving your spec-switchers offspec loot leaves you undergeared for harder encounters -- you really don't need to have that many more than 25 raiders. Add in a few to cover attendance given your usual no-show rate, or recruit people without lives who will make every raid, and Bob's your uncle.
However, summer changes things. For some, the summer holidays simply mean glorious uninterrupted time off work to farm, catch up on quests and level those alts -- but for many others, they mean trips abroad to strange places without Internet and without WoW-capable PCs. If you're running a roster that's barely above the raid size to start with, losing several people for weeks at a time to holidays can be devastating, especially if you're trying to beat new encounters.
The obvious solution is to recruit, but that in itself causes problems. In the short-term, there can be tension and conflicts as you overfill with trials, leading to disgruntled raiders who feel their spots are being taken. (However, there is a bonus here in that some of your raiders will be quite happy to give up a spot for bosses they don't want any more!) There's also the possibility that new trials won't pass, leading to an endless succession of new recruits and frustration on the part of the raid leaders, or fellow raiders in the same class/role, having to explain the fights multiple times over.
In the long-term, of course, it's very hard to strike the right balance. If you recruit sufficiently to still have some wiggle room in the roster at the height of the summer holidays, when people come back from their trips you'll suddenly find an overfull bench -- which can be very hard for people to deal with. However, one side-effect of summer attrition is the relatively high number of people who tend to stop raiding altogether, so this isn't quite as terrible a problem as it might sound at first.
The turnover caused by summer all contributes to a usually-discernible change in the atmosphere and feel of a guild. People return from breaks and have no idea who half the names on the guild list are; officers struggle to manage internal dissension and accommodate prima donna veterans who waltz back in after long breaks and expect loot; raiders get frustrated at new trials who don't automatically know everything.
How to best manage this?
The most important thing to do is really to acknowledge that summer happens. People will vanish, come back, and you will end up with pre-planned raid nights where you simply don't have enough people online to put a raid together. If you deal with all of this as it happens, it can be frustrating, so there are various discussions the officers (and guild as a whole) can have beforehand to ease some of the stress.
Firstly, it's useful to lay down rules on communicating absences so you have some idea of who will be away over the summer, and for how long. From this you can figure out where your trouble spots are likely to be, and decide whether to actively recruit to expand the roster, or simply call a raiding break.
Many guilds successfully take time off during the summer months due to core raider absences and it works really well for them. Overtaxed raiders get time to kick off and rewind, and in some of the more regimented guilds, being told about a break beforehand means they can plan their own summer trips knowing there's no latent WoW guilt to deal with. For other guilds, breaks can be devastating -- the mere suggestion of a break instantly divides the raidforce into those who want a bit of time off, and those who think even sleep is not an option for the hardcore. This summer comes at an interesting time raid-wise, where guilds are still progressing through Ulduar and working on Algalon and Yogg-0; the concept of delaying these kills is alien to some raiders.
If you decide that you want to recruit to cover the low period then it's advisable to start early so you can cherry-pick trials rather than take the first few that come along just to make up the numbers. If you don't usually recruit or have many trials, and you're planning to take in more than normal, it's worth explaining to the guild -- or at least specifically affected players -- so they don't take it as a sign of something worse than just covering summer.
It's also worth having an internal discussion about what your officers and raiders feel is the right thing to do if you haven't got a full roster, or you have a sub-par composition. Would they rather come back another night with a perfect setup than wipe, or do they want to give things a go and maybe triumph against the odds? Are people likely to welcome a 10-man evening rather than do easy bosses with 23 people?
Making these decisions again and again on the fly can be draining, so it really helps to have some idea of what people actually want to do -- after all, you run the guild for everyone, not just for yourself. As a raid leader, in the past I've called off a raid or two due to feeling hot, exhausted and with a burgeoning headache, and feeling I would rather spend the evening in a cool bath than wiping with an imperfect setup. I learned from this that despite my own preferences, the guild would have been quite happy to wipe all night rather than have cancelled their barbecues and summer parties for nothing.
The key thing is to know your guild, to plan ahead, and to acknowledge that sometimes people do just want to relax in the shade rather than play WoW. A strange concept, admittedly. Being flexible and accommodating to those with summer absences is far more likely to keep the guild healthy in the long run than reacting badly to vacations. Just be prepared for internal changes and drama, as always!
Image from CPG Midsummer Social by ulalume