No. Sin requires both knowledge and consent. If there is no knowledge or no consent, then there is no sin. Had you known it was Friday and deliberately chosen to eat your salami sandwich anyway, you would have been in mortal sin. If you forgot, you forgot, and to truly forget (as distinguished from false “forgetfulness”) is not a sin.
That said, perhaps you can use this mistake as an opportunity to plan better next time. If you know your schedule makes it likely you could forget what day it is, then put together your Friday meals ahead of time when you’re awake, mark them “Friday,” and put a reminder on your refrigerator on Thursday night that you need to eat your Friday meals.
Too many people do honestly forget about obligations, but do not take precautions to make sure such forgetfulness doesn’t happen again. If, after you’ve forgotten once, you deliberately choose not to plan ahead again and make the same mistake again, there could be some culpability for that choice not to prepare for your Lenten observances.
