There is usually a flurry of activity at the top of each hour when many of the m/s expeditions check 160m for 10-15 minutes or so. Those periods are generally more productive than the rest of the time, though with the increasing numbers of operators contesting on Topband, and the importance of its multipliers to a final multi-band score, there is generally some activity through out the hours of darkness.
The New England operators can hear Ireland from around 21:00 or 22:00 UTC, and will be hearing Europe all the way to about 6:00 UTC, probably around their sunrise. Best times to Europe from the US generally are from about 0000Z to 0730Z, with the British Isles and Scandinavia a bit later perhaps. A couple of years ago, they were still hearing SM's at 1040Z!!! As is well known, sunrise/sunset on either side is the best time to work anyone on 160m, so these times make sense. The times will, of course, favor the sunrise grey line in Europe.
Here's the 160m log from a well-known US multi/multi station during CQWW, which will give an idea of the times and openings they were getting. Its been re-formatted in time-order. The antenna was a 5 element (3 active elements) vertical array, and the transmit power was 1500W.
31-Oct-99 0001 184 1.839 IG9A IG9
30-Oct-99 0005 8 1.833 PJ4B PJ2
30-Oct-99 0006 9 1.8 KP4AAQ KP4
30-Oct-99 0006 10 1.8 V26B V2
30-Oct-99 0008 11 1.8 J3A J3
30-Oct-99 0012 12 1.8 VY2SS
30-Oct-99 0015 13 1.8 YV3AZC YV
30-Oct-99 0018 14 1.8 ZF2LA ZF
31-Oct-99 0022 186 1.849 IH9P
30-Oct-99 0030 19 1.8 ZX0F PY0F
31-Oct-99 0042 189 1.814 CT1EEB CT
31-Oct-99 0120 191 1.8 TM1C F
30-Oct-99 0155 38 1.8 VP9BO VP9
31-Oct-99 0213 200 1.8 EA8BH EA8
30-Oct-99 0243 53 1.8 PJ8/N7KG PJ7
31-Oct-99 0305 211 1.8 VY2OX
30-Oct-99 0319 62 1.833 CN8WW CN
31-Oct-99 0324 212 1.836 CQ9K CT3
30-Oct-99 0405 83 1.838 FG5BG FG
30-Oct-99 0412 90 1.8 K6SE
30-Oct-99 0415 91 1.835 YV2IF
30-Oct-99 0424 94 1.837 VO2CQ
30-Oct-99 0444 105 1.8 VP2MCS VP2M
30-Oct-99 0450 109 1.834 NP2B KP2
31-Oct-99 0507 226 1.8 EA4ML EA
30-Oct-99 0511 110 1.835 KH6CC KH6
30-Oct-99 0516 111 1.8 XE2/N7RK XE
31-Oct-99 0528 227 1.8 DL7ON DL
30-Oct-99 0535 115 1.8 VP5T VP5
31-Oct-99 0543 228 1.8 SP3GRM SP
30-Oct-99 0550 120 1.842 HI9/DK8YY HI
31-Oct-99 0551 229 1.8 OK1DIG OK
30-Oct-99 0605 122 1.8 8P1A 8P
30-Oct-99 0607 123 1.8 HC8A HC8
31-Oct-99 0611 230 1.847 J6J J6
30-Oct-99 0700 128 1.8 P40E P4
30-Oct-99 0702 129 1.8 XE2DV
30-Oct-99 0704 130 1.8 V47KP V4
31-Oct-99 0705 234 1.8 TI5EBU TI
30-Oct-99 0807 134 1.827 6D2X
30-Oct-99 0810 135 1.825 8R1K 8R
30-Oct-99 0823 140 1.825 PJ2C
30-Oct-99 0829 141 1.84 FS/K7ZUM FJ
30-Oct-99 0834 143 1.8 XE2DN
30-Oct-99 0853 146 1.8 VP5DX
30-Oct-99 0912 148 1.847 FM5BH FM
30-Oct-99 0912 149 1.832 VP5R
31-Oct-99 0914 241 1.838 VK3AJJ
30-Oct-99 1137 177 1.8 VK5GN VK
31-Oct-99 2325 264 1.8 OX3LG OX
31-Oct-99 2333 265 1.8 S57M S5
On phone, 1830-35 is supposed to be used for intercontinental qso's, but it often degenerates quickly and on ssb as there is really only room for 1 or 2 pileups in there anyway. Any frequency you can legally operate on between 1800-1900khz will be fine when working into the States; much higher than that and many operators may not find you so easily.
The most activity (and qrm) is usually found from about 1825-1850khz. Some countries still have smaller band restrictions so you have to dial around and work some of them where they call cq and not expect them to all come to you... changing cq frequency occasionally is also a good idea as some regions have qrm from other services that may interfere, and again some countries restrict the band more so may not be able to respond to you just anywhere on the band.
Note that not all countries have the same frequency allocations, or even allocations that overlap. For instance, Finland has 1810-1850Khz, and Japan has 1907.5-1912.5Khz. So, for an OH station to work JA, he will need to operate split-frequency, listening out of his normal allocation.
When calling CQ, you should announce that you are listening on your TX frequency and also out of band, on the other country's TX frequency.
Tom, W8JI, advises: I often hear dozens of Europeans in any SSB contest on 160 below 1850, and without fail they are under strong USA stations parked on one frequency and working only domestic contacts.
I'm sure most don't realize that makes working DX simplex on 160 during contests impossible. It is just silly to work simplex there, especially with so many wide signals and people who won't let someone else who hears the DX have a shot at it.
If the DX stations would listen up the band (USA can transmit anywhere from 1843 up to 2000), and the USA stations would stay above 1843, everyone would be much better off.
Note: A full list of World frequency Allocations can be seen on this website here.
It is useful to know when the band is open, and whether your equipment and antennas are set up correctly. If amateur stations are not available, then commercial beacons can be used to assess conditions. Here is a list of the more commonly used ones, as posted to the Topband Reflector: