"Tomorrow do thy worst" is a challenge thrown down like a glove. The speaker addresses the future as if it were an opponent, and he does it from a position of strength. The lines come from Dryden's English version of an ode by the Roman poet Horace, which may explain the tone: a calm, squared-shoulder composure in the face of whatever fortune sends. The central claim is that real happiness belongs only to the person who can say at the end of a day, "I have lived today." Everything else, such as money, safety or plans, can be threatened. A day fully lived cannot.
The poem argues this through the language of ownership. The happy man can "call today his own." The joys have been "possessed" and are "mine." By the end he can say "I have had my hour," as if holding a receipt. Happiness here is not a mood but property, and fate becomes a kind of creditor who arrives to repossess and finds nothing he is allowed to take. Even the phrase "secure within" sounds like a locked house or a fortress, with the speaker safe behind his own walls.
The imagery is sparse but pointed. "Be fair or foul or rain or shine" reduces every disaster fate might invent to the weather, and the light, sing-song rhythm of the line helps. The threat sounds almost trivial, something you can put up an umbrella against. Then the poem turns bolder: "Not Heaven itself upon the past has power." It is a startling thing to say, since it limits even divine authority. The final line closes the argument with a tautology, "what has been, has been," where the repeated phrase clicks shut like a door that no one can reopen.
The form supports this confidence. The poem runs in rhyming couplets (alone/own, say/today, shine/mine, power/hour), and the rhymes often link the speaker with what he claims. "Mine" echoes "shine," and "say" is tied to "today." The last pair is the most telling: "power" is rhymed with "hour," so Heaven's power is set directly against one human hour, and the hour has the last word. The line lengths are also deliberately uneven. The poem opens with a full ten-syllable line, then drops to shorter lines of about eight syllables, then stretches out into long twelve-syllable lines that finish three of the couplets ("Tomorrow do thy worst, for I have lived today" is the first). Each long line works like a verdict, with the sentence swelling outward before it lands. The last line is the longest in feel as well as in length, as if the speaker were savouring his hour.
One subtle detail is the grammar. "I have lived today" is in the perfect tense, which treats the day as finished even while it is still being spoken. The security the poem celebrates comes from turning the present into the past, where nothing can touch it. The happy man does not simply enjoy his day; he banks it. This is a slightly different idea from the familiar advice to enjoy life while it lasts. Here the point is that a joy, once had, will never stop having happened.
A thoughtful reader might push back. The speaker counts only "the joys I have possessed," and sorrows quietly disappear from the account. It is also easier to shout at tomorrow when today has gone well. But part of the poem's appeal is that it does not pretend to be gentle or balanced. It is a defiant boast, and its compression gives it force. In eight lines it holds a whole attitude to time: confident, a little proud, and unwilling to let the future spoil the present. That makes it memorable, and easy to carry in your head on a bad morning.