It's not that unusual. Here's when memberships were launched in previous years:
2018: 19th March
2017: 9th March
2016: 6th April
2015: 9th March
2014: 22nd April
2013: 26th April
2012: 14th April
2011: 4th April
So, while it's in the early part of the window that it's been announced in the past, it's certainly not the earliest we've announced Membership details...