<em>Weak</em> is an adjective meaning lacking strength, power or conviction: "the coffee was weak", "a weak argument". <em>Week</em> is a noun meaning a period of seven days: "I'll call you next week". If you can swap in <em>seven days</em>, write <em>week</em>; if you can swap in <em>feeble</em>, write <em>weak</em>.
The core difference: adjective vs noun
These two sound identical in most accents but do completely different grammatical jobs, which makes the choice easy once you look at the sentence structure rather than the sound.
Weak describes something: a weak pulse, weak tea, weak sales, the weakest link. It has comparative forms (weaker, weakest) and a noun form (weakness) and a verb form (weaken).
Week is a unit of time: a working week, three weeks' notice, week after week. It can be counted, so it has a plural, which weak never does. If you have written weaks, the word is wrong.
The test you can run
Replace the word with seven days. See you next seven days is clumsy but comprehensible, so week is right. Now try replacing it with feeble or not strong: a feeble excuse works, so a weak excuse.
The spelling hook is straightforward: week contains ee, and so does seven days' partner word weekend — plus Wednesday, weekday and weekly all keep it. Weak shares its ea with break and tear, things that give way.
Where the error shows up
Almost always in sign-offs and scheduling, where people type fast:
- Have a great week — not a great weak, which wishes someone seven bad days.
- Let's push it to next week.
- A weak signal, a weak password, weak evidence — all adjectives, all ea.
Because both spellings are real words, no spellchecker will flag the mistake. It is worth a targeted search before sending anything important.
The compounds and the edge case
The compound forms follow the base word without exception: weekend, weekday, weeknight, midweek, fortnight (two weeks) all take ee; weaken, weakness, weak-willed, weak-kneed all take ea.
Watch the apostrophe in time expressions. Periods of time take a possessive: a week's holiday, two weeks' notice. The apostrophe moves after the s in the plural, and leaving it out entirely is the commonest punctuation slip attached to this word.
Finally, note weak has technical senses — the weak nuclear force in physics, weak verbs in grammar — where it does not mean feeble at all.
Frequently Asked Questions
Is it "have a good week" or "have a good weak"?
Have a good week. You are wishing someone a good seven days, so it takes the noun spelled with ee.
Is it "two weeks notice" or "two weeks' notice"?
Two weeks' notice, with the apostrophe after the s. Periods of time take a possessive form, in the same way as "a day's work" or "a week's holiday".
How do I remember which spelling is which?
Week shares its double e with weekend and weekly, which are all about days. Weak keeps its ea like break — something that gives way under pressure.