a week tomorrow/on (some day)/etc.
One week from the day specified. Primarily heard in UK. I'm flying to Ireland a week on Saturday for my brother's wedding. We need that report finished a week tomorrow.
a week yesterday
A week and a day before today. I last saw Stu a week yesterday, before he left on vacation.
a week yesterday/last (some day)/etc.
One week before the day specified. They only gave me the assignment a week yesterday, so I'm really stressed out about getting it finished by tomorrow. Chris left on his work trip a week last Tuesday.