ANSWER:

You will be rewarded in Sha Allah for your intention. If you had a habit of fasting it every year in the past, you will be rewarded as if you had fasted it in Sha Allah 

QUESTION: Sheikh, I really wanted to fast on the Day of Arafah this year, as I learnt about the Hadith that it expiates the sins of the previous and the upcoming years, but I got my periods, which was decreed for me by Allah. Will I get the reward, will my sins be forgiven (if I verbally seek Istaghfaar) while I cannot fast?