If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Sillian: The hottest months are usually July and August, with an average temperature of 12°C, while the coldest months are January and December, with an average of -4°C. The snowiest months in Sillian are February, January, April and December, with each month seeing an average of 361 cm of snowfall.