Here's the success ratio for each month for 2012 to present. Total is the number registered to start in that month, success should be obvious.

+-------------+-------+----------+
| start_month | total | success |
+-------------+-------+----------+
| 1 | 85 | 77 |
| 2 | 4 | 3 |
| 3 | 8 | 4 |
| 4 | 56 | 30 |
| 5 | 246 | 132 |
| 6 | 554 | 302 |
| 7 | 398 | 185 |
| 8 | 231 | 99 |
| 9 | 134 | 46 |
| 10 | 28 | 7 |
| 11 | 2 | 1 |
| 12 | 28 | 4 |
+-------------+-------+----------+

Ignore January and December, there's winter rounds in there plus some where the data isn't complete and defaults to 1st January.

You need to be wary about looking at the above too simplistically, there have been years when there's been very few successes in say June because of bad weather and people just didn't set off. Over time things do even out, you aren't going to get the first weekend in June always being horrendous weather for example.

The "core" months of May, June and July have 617/1198 successes or just over 50% which is pretty much the overall success rate, but hardly surprising since those months dominate. April, August and September are very hit and miss, mostly due to weather plus the shorter daylight hours.

Generally just register for a date that you think might be OK, it's not fixed in stone - you can always change it. Registration just lets us know what to expect in terms of forms/new members and puts a mark next to your proposed start date/time on the website so that others can avoid should they so wish.