I had a similar issue during the summer here in Greece, with ambient temperatures of about 30-35 C. I went away on a holiday and I did not want to leave the air-conditioning on all day (and thus double my electricity consumption) so I had a setup like this:
- A cgminer process running with intensity 1 (-I 1) 24 hours a day. This produced around 350 MH/s on a 5870 and acceptable temperatures for the cards.
- A cron job starting a second cgminer process with intensity 3 (-I 3) during the night (using the --sched-start and --sched-stop switches), when ambient temperatures were lower. When that process kicked in, it was going to 320-330 MH/s while the first one reduced to around 110-120 MH/s, giving me an effective hash rate of about 440 MH/s which is the same like what I got running a single cgminer with intensity 9
- The clocks were set the same all the time (atitweak -e 950 -m 319 -v 1.08)
It should be simple to implement.
If you actually want to completely turn off some cards for certain day intervals, as your post suggests, then just omit the first cgminer process.
Hope this helps.