According to nookipedia, the U.F.O. is supposed to appear on Monday till Thursday and within the first ten minutes of an hour.
I'm very skeptical about these claims and want to know if this is indeed true?
That's when it
can appear, but it won't be every hour, or every day, or even every week.
When I'm UFO hunting (which isn't often any more, since I've already collected all of the Gulliver items), I first try to see if the current day is an ideal day. This is how I check:
1. First of all, avoid being outdoors in the first ten minutes of an hour to be sure you don't miss it before you're ready.
2. Ask Booker if any visitors (Saharah, Wendell, etc.) are in town. If yes, then I don't even try that day. While I've heard that the UFO may still come with a visitor in town, it makes it harder to know for sure so you would likely be wasting your time.
3. Play during the half-hour that Pete is supposed to be delivering mail (9-9:30 am or 5-5:30pm) and search your town thoroughly for him. If you see Pete, then today is not a UFO day. If you search thoroughly and you are
sure he is not anywhere in town, then today is a UFO day. (Assuming all previously met conditions are right--no other visitors in town, Monday thru Thursday, etc.).
If you're serious about hunting UFOs, I strongly suggest placing patterns where the UFO can come. There will be three possible paths (horizontal lines at the top of the 2nd, 3rd, and 4th rows of acres). This way, you'll know exactly where to run if you spot the UFO.
Once you know it's a UFO day, wait until x:02 (it helps to have a watch/clock synchronized to the Wii clock) and run up and down the center of town (from the top possible UFO path to the bottom one). If nothing by x:04, you can relax until x:07. Then repeat until x:09. If you hear or see it, and can tell which way it's going, run to your pre-marked spot and shoot him down. If no luck, try again the next hour.
If it comes and you miss it, you can reset and try again the next hour. Just avoid starting up too close to the spawn times at x:02 and x:07, like laineybop mentioned. Tracking Gulliver down takes patience and persistence (unless you just get lucky--but that won't happen very often). Good luck!