Forgetting what day of the week it is may be the result of associating Monday with misery and Friday with fun, say psychologists.
The two ends of the working week both have strong identities, pushing "non-descript" Tuesday, Wednesday and Thursday to the back of our minds.
Asked what words they strongly associated with different days, study volunteers attached more mental representations to Monday and Friday.
While Mondays mainly prompted negative words such as "boring", "hectic" and "tired", Fridays were associated with positive words including "party", "freedom" and "release".
Almost 40% of the participants said they sometimes confused the current day with the previous or following day, mostly during the middle of the week.
Even more confusion reigned during a Bank Holiday week, with people often feeling they were a day behind.
Lead researcher Dr David Ellis, from the University of Lincoln's School of Psychology, said: "The seven day weekly cycle is repeated for all of us from birth, and we believe this results in each day of the week acquiring its own character.
"Indeed, more than a third of participants reported that the current day felt like a different day, and most of those feelings were on Tuesdays, Wednesdays and Thursdays, reflecting the midweek dip in associations attached to different days.
"Our research implies that time cycles can shape cognition even when they are socially constructed. The Bank Holiday effect implies that apparent weekday is not determined solely by the seven-day period of the weekly cycle: transitions between the working week and weekend also play a role."
Co-author Dr Rob Jenkins, from the University of York, said: "One reason behind midweek days evoking fewer associations than other days could be down to how infrequently they occur in natural language, thus providing fewer opportunities for associations to form. For example we have an abundance of pop songs which make use of Mondays and Fridays, while the midweek days are rarely used.
"If links can be made in the future that aspects of behaviour such as risk or tolerance also vary systematically over the week, the implications could be profound, not only for individual behaviour, but also for psychological measurement."
The findings appear in the online journal Public Library of Science ONE.