Deworming is a complicated subject, and best tackled with the help of your veterinarian. But there are some seasonal concerns. The most important thing is to use moxidectin or ivermectin in December or January, which will kill encysted small strongyles and immature worms. But it will also kill bot larvae, which over-winter in the horse.
A baseline program deworms your horse every six months with a product that is effective against migrating large strongyles (ivermectin, moxidectin or fenbendazole at a double dose for five days). Including praziquantel in one of the doses also supplies tapeworm control. In most climates the optimal time for semiannual deworming is spring and fall. This will eradicate large strongyles and give adequate control of tapeworms, and it probably provides all the protection needed by up to 50% of your herd. Ask your veterinarian to conduct a fecal egg count test to identify which horses are more susceptible to infection with small strongyles. Those horses can be treated more often, but the timing of extra treatments is very critical. Horses in the northern states become infected during spring, summer, and autumn. That's when your extra treatments should be concentrated, with less concern in the winter. In comparison, horses in the southern states become infected in the autumn, winter, and spring but summer treatments can be eliminated because the weather is too hot for worm transmission.