Thursday will be your travel day to the location of the race. While most check-ins take place the day before the race, be advised that some check-ins, especially for full Ironman races, only take place two days before (Thursday evening for a Saturday race). In this case there may not be an option for a night-before or morning-of check-in, and if you miss it, you can’t race. Most race websites have a document with very detailed and informative descriptions of these details. Read it several times until you are familiar with it.
Upon arrival, it’s a good idea to do a short bike ride to get the blood flowing after sitting in the car or plane, and this is a necessity if you had to reassemble your bike after travel. You don’t want to wait until race day morning to find out something is wrong with it. Many larger triathlons have “villages” at the check-in point where it’s possible to get a bike tune-up if you need it, and also to purchase spare tubes, CO2 cartridges, gels, etc. Note that while it may be possible to put CO2 cartridges into checked luggage on an airplane, it will depend on the airline and is generally recognized to be a bad idea. There’s no need to submit yourself to the possibility of extra scrutiny at the airport when cartridges are always available at the race site for a few bucks each.
Try to get a good night’s sleep tonight, as it may be your last full night of sleep before the race.