At the end of the season, all leftover seasonal candles will be converted to regular candles in a 1:1 ratio. When you log in for the first time after the Season ends, you should see a popup similar to what is pictured below.
For Example:
If you have 15 Seasonal Candles at the conclusion of the season, you will receive 15 regular Candles and the Seasonal Candle counter will disappear from the User Interface.