If the central idea of "The Masque of the Red Death" is to live life like it’s your last day, what quote from the story would support that?
A really good bet for this theme would be the section which describes the clock. It's longer than I will include here, but these parts convey the information most crucial to your idea:
There stood against the western wall, a gigantic clock of ebony. Its pendulum swung to and fro with a dull, heavy, monstrous clang: and when the minute-hand made the circuit of the face, and the hour was to be stricken, there came from the brazen lungs of the clock a sound which was clear and loud and deep and exceedingly musical . . . the musicians of the orchestra were constrained to pause, momentarily, in their performance, to hearken to the sound; and thus the waltzers perforce ceased their evolutions . . . and then, after the lapse of sixty minutes, (which embrace three thousand and six hundred seconds of the Time that flies,) there came yet another chiming of the clock, and then were the same disconcert . . . as before.
In this passage, the party goers are reminded in a very audible way that time is passing them by. With every tick of the clock, they inch closer and closer to their deaths, which they presume is at some point in the very distant future. However, the end is much closer than they presume. Likewise, each of us has a silent clock ticking away in the background of our own lives. It's important that, like the dancers and musicians, each of us pauses to reflect on this passage of time periodically and does not simply keep dancing our way toward death.
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