Yaelqes Hello, 
Welcome to Israel of Sri Lanka. 
Yeah you can visit Sri Lanka at any given moment, no matter when, you will have so much to do, so many things to see. December is the rainy little country to the east coast as Kandy and nuwaraeliya. So you can see more fog and colder but it also gets more beautiful naturally. But down south such as Galle and Hikkaduwa, it's very sunny most of the time and you see many beach activities going on. This is why I mentioned about no matter what time it, you will have a lot of experience to gain. In April, there is a big festival happening in the name of "New Year festival" and related culture. 

"South West monsoon which mainly affects the southern regions, western and central during the months of May to July, while the north-east monsoon brings rain in the north and east from November to January". 
attention: Http://www.cyriltours.com/srilanka.php 

However, considering the weather, you can choose your goals according to your preferences. If you want to be where it is cold and foggy, I suggest you take a trip to the Land and if you want to have sunny days and warm climate, southern and western coast is the best options for you. 

If you travel to Israel, you can choose to visit the ancient cities like Anuradhapura, Polonnaruwa and some great things like Siguiri, Dambulla..etc 

Even if you love wildlife then there are many places to visit such as Yaala, Wilpattu or even Sinharaja rain forest. Bird watching and trekking is a joint operation and even biking and climbing is also possible. 

Besides the seasonal period of man's climax in early December through May. 

So there are a lot of activities and all you need is to see what things you can do and what you want. And plan for those with the time frame that you have. 

Hope this information is helpful. Thank you.

 

Apologize if the language is not correct. Thank you.