If we take an example of fasting on day of Muharram,When the Prophet (peace and blessings be upon him) arrived in Madinah in 622 CE, he found that the Jews there fasted on Muharram 10 and asked them the reason for their fasting on this day. They said,” This is a blessed day. On this day Allah saved the Children of Israel from their enemy (in Egypt) and so Prophet Musa [Moses] fasted on this day giving thanks to Allah.”
The Prophet (peace and blessings be upon him) said,
“We are closer to Musa than you are.”
He fasted on that day and commanded Muslims to fast on this day.
Also by considering following headees,
Narrated ‘Umar bin Al-Khattab Radi Allahu Anhu: The Prophet Sallallahu Alaihi wasallam said, “”The rewards (of deeds) are according to the intention, and everybody will get the reward for what he has intended. So whoever emigrated for Allah’s and His Apostle’s sake, his emigration was for Allah and His Apostle Sallallahu Alaihi wasallam ; and whoever emigrated for worldly benefits, or to marry a woman, then his emigration was for the thing for what he emigrated for.””
Sahih Bukhari, Vol 7, Book 62, No. 8″
So by keeping this thing in mind,if one eats pancake (just because they are cheap) then he/she should not have intentions of following Christianity.
As you said you are eating just because they are cheap on that day ( not following Christianity ) then you are good to eat.