Agree with all above.
IF - this is an "if" - you have decided that REITS do fit within your investment planning, and if you are considering a relatively large amount or percentage ("relatively" being the operative word - it doesn't need to be a big percentage of your total portfolio), you might feel more comfortable stretching the purchase over several weeks or months.
This is mostly for your emotional comfort, especially if this is the first time you are investing in REITS. They are incredibly volatile, and with larger swings that many other categories. How would you feel if you invested the full amount at one time, and it goes down 5% (or more) that very day? Probably won't feel good if you aren't also used to the upward swings, which could take a while to occur (and are never guaranteed).
So perhaps 1/3 or 1/4 over 3 or 4 weeks or months. Choose a day/date in advance, or wait until just after a drop (which might not occur in your time frame, so think this strategy through). The easiest might be to choose a few dates in advance, and "just do it".
IF this investment is for the very long term - and there's probably NO good reason to invest in REITS for any short term goals, unless you have a very special crystal ball [in which case, please call me at 555.555.5555] - then even the wild swings in the short term will *probably* work out okay.
But the "down" trend COULD go on for years, so if you decide to do this, also decide NOT to bail out after a major/extended downturn.
Otherwise, that's classic "buying high/selling low" - and others will be waiting to buy on those major downturns, and thus wait it out.