Weather is definitely something that can have an effect on people. I do not have fibromyalgia, and even I find that sometimes I simply don't have the energy that I would like during hot weather, and I know that my relative who has the condition struggles even more. I don't know how she manages to get through the day sometimes, because even I feel as though the only thing that I would like to do is to have a lie down in bed.
If you know that there is going to be warm weather, then you should do everything that you can in order to keep cool. For example ensure that you drink as much water as you can, and keep the area that you're sitting in nice and cool. If you work in an office, it is a good idea to ask your manager whether you're able to have a fan at your desk, or perhaps if you're able to sit nearer to the air conditioning if you have any. People should try to help you, so don't be afraid to ask if you feel as though you need to.
If you know that you struggle when it's cold, then make sure you wear plenty of layers so that you should be able to cope, and set the heating to come on in your home a short time before you expect to return, so that it can have kicked in by the time you get home.