Here’s a list of some online events for International Women’s Day 2021 – quite varied but not that many gender critical ones. But I suppose if we don’t organise them they don’t happen!
www.womensgrid.org.uk/?p=14393
On one of the threads about IWD we talked a bit about whether to have one or maybe two special IWD threads on FWR to make some public positive statements. Ideas were for a thread about amazing mums, aunts, grans, sisters … And/or women who have inspired us … Or ….
www.mumsnet.com/Talk/womens_rights/4179157-IWD-2021-what-can-we-hope-for-this-year?pg=2
Just to add a point of history. The web site that says it is the “official” IWD web site is in fact run by a PR company, ie as a sales gimmick. What is so sad, but a lesson to be learnt, is that in about 10 years they have managed to get not just individuals but lots of organisations and even Government departments to accept that it is the official site. So if you see anyone using the Challenge slogan they are a victim of successful commercial advertising. 
If there is an “official” theme any year it might be the one set by the UN Women who after all got 8 March acknowledged as IWD in 1977. www.unwomen.org/en/news/stories/2020/11/announcer-international-womens-day-2021
There are a number of versions of the origins of IWD. Here is one! www.womenaid.org/events/iwd/iwdeverywomen.htm
Reclaim IWD for Women and Women’s Sex Based Rights!