When a farmer needs water, he likes rainy days. When friends go travelling, they like sunny days. These are ok . But, it is more correct to say these : When a farmer needs water, he hopes for rainy days. When friends have to travel, they hope for sunny days/they prefer to do it on sunny days . "When " is used instead of "if" to show the something you will normally do at that time when it happens . When a farmer needs water, he likes rainy days. > Does he not like it when he has water ? Farmers like rainy days because they can have a lot of water.

Your first sentence, 'When a farmer needs water, he likes rainy days' is correct. However your second sentence is not. Instead of saying 'When friends have a travel, they like sunny days.' You should say 'When friends travel, they like sunny days.'. This is because the word 'Travel' is not countable, so you usually shouldn't put 'have a' in front of it.

Thank you, Mojave!

Travel is not a countable noun, so we never say "a travel" and we never say "have a travel" unless maybe it is part of a bigger clause. We simply say "travel". When friends travel, they like sunny days.