Almost every software development deadline is movable. Even if the CEO of your Fourtune 50 company has promised via international press conference your product will ship August 1st, the deadline can move.
Of course, your CEO will scream at you and be pissed, the public might be disappointed, people may be fired, but if your company set the deadline, your company can move it.
Compare that to a deadline that is IMPOSSIBLE to move, no matter how much you or your company wants it.
If your product release is tied to a national holiday, astrological event, or similarly externally imposed temporal happening, be very careful how you build.
Be paranoid. Build, test, and deploy only the bare essentials. Start immediately. There’s always time to add on further enhancements once you have something live.
Need to build a home search site in time for a local leather festival’s parade? Start with a static HTML page that lists all the properties in alphabetical order.
Need a site that lets people track an upcoming NASA mission? Just publish a static timetable of the launch events. Build the interactive streaming video stuff once that’s up.
Having zero features at the deadline is utterly unacceptable, so get something greater than zero deployed ASAP. Remember that any number of catastrophes can happen between now and your deadline (the one you can’t miss), so get it out of the way.
This also has the nice side effect of acting as a kind of minimum viable product, and you can get early-users’ feedback to help drive the other features. It would suck to spend months building an augmented reality feature for 11/11/11 day, only to find out three people used it.