I believe that the first named snowdrop I ever bought was Galanthus elwesii 'Peter Gatehouse'. I was at my first Galanthus Gala in 2001 and it was one of snowdrops left behind on the sales tables after the scrum had subsided and I could get near. It came from Marchants Hardy Plants and cost me £7.50. It has done well for me over the 16 years since and so I now have little groups in various locations around the garden. These can be weeks apart in their flowering time. The ones that flower the latest seem to be in areas that are driest during the summer. The soil in my garden can get bone dry in summer so the autumn rains may take some time to raise the moisture level. I suspect that a certain level of moisture is a prerequisite to flowering. It is an early season this year and I estimate about 50% of my Peter Gatehouse are showing now.
I grow a lot of snowdrops in plastic pots. I do this to ensure that they are healthy before being planted out in the garden but space in the garden is at a premium so they often spend longer in pots than I might wish. Pot-grown autumn snowdrops almost always flower earlier than those planted-out in the garden. I suspect that the soil in the pot retains more moisture when it rains because the moisture cannot get deep into the ground as it can in the garden. And the temperature of the soil in the pot, surrounded as it is by air, will cool faster than the soil in the garden.
Comparing the pot-grown snowdrops (and excluding those new ones that have not settled) 'Remember Remember' always flowers before 'Peter Gatehouse'. My pot of 'Remember Remember' has finished flowering now whilst the ones in the garden are about to open. 'Hollis' seems to flower at about the same time as 'Peter Gatehouse', possibly slightly earlier. This year I have one called 'Santa Claus' that has also finished flowering already but I have not had that one long enough to know if this will be consistent. My 'Barnes', which I only grow in the garden, came from the garden of 'Richard Ayres' and typically flowers late November. But I was talking to Rod Leeds about a month ago and he said his 'Barnes' was almost over and flowers in September or October every year.