There is a mixing of two separate issues here, mortal sin with negligence.
Mortal sin requires grave matter, full knowledge, and deliberate consent. If one of those is not present, there is no mortal sin. Although it appears that the first two conditions were present in your situation, the third apparently was not because you said that you did not deliberately choose to miss Mass. According to your report, you missed Mass through negligence, not through a deliberate decision not to go.
Which brings us to negligence. Yes, it does appear that if you intended to go to Mass on All Saints Day but did not take the time to check the schedule of Masses, you may have been negligent in properly preparing to meet your Catholic obligations. Simple negligence though, ordinarily speaking anyway, is not a mortal sin. The Church does not require the confession of venial sins, but it does strongly recommend it. If you like, you can mention this the next time you go to confession. Doing so can open you to the grace to be more diligent about meeting your obligations as a Catholic.
