Curiosity prompted by being a minority that lives in the Southern hemisphere. Are we forgotten, overlooked, ignored or simply hated by those in the temperate latitudes of the North who would actually consider 'now' to be winter ;-)
Trying to answer the specific answer on what percent of SE users are in 'winter' during 'winter bash', I created this query that gets the number of users in the Southern Hemisphere (this is only for StackOverflow btw), and the current count is quite low, which is (at the time of writing this answer) 65212, whilst the number of users that are NOT in the Southern Hemisphere are 2362818, which means that the percentage is:
Of course we are not taking into account the users that didn't list their countries in their profiles (which is quite high), but this is a good sample.
We can also run the same query on all the SE sites to get a better estimation.
I hope this answers your question.
Luckily this year you can at least customize the homepage of https://winterbash2018.stackexchange.com/. Here's what I knitted in order to test the knitting editor while implementing it:
The Developer Survey 2018 contains geography information, including monthly Stack Overflow visits:
I've added the countries lying (predominantly) on the Southern hemisphere and came up with a percentage of about 7%, so for them 'Summer Bash' would be more appropriate. The other 93% can still enjoy 'Winter Bash' with no lexical issues.