Missing Mass on Sunday is gravely immoral and thus would ordinarily be mortally sinful. If you can’t possibly make it to Mass b/c you are sick, just gave birth, were sent on an unavoidable work trip to a place with no Catholic parishes nearby, etc. then you wouldn’t be giving full consent and it therefore wouldn’t be mortally sinful.
Missing weekday Mass is not sinful as you are under no obligation attend Mass except for on Sundays and Holy Days of Obligation.
Going to Mass on Monday instead of Sunday doesn’t make up for missing your Sunday obligation.
To fully answer your question, we’d have to know why you can’t make it on Sunday (or Sat evening). Perhaps that is something you could speak to your priest about at your next confession.
God bless.