May 1 is International Labor Day in the rest of the world, except US , as US has it sometime in September or so So, expect lower trade volumes on Monday, May 1.
Japan - Today no Labor Day holiday. Today don't bother to trade the Asian and European sessions. The worst time to trade is around Christmas & New Year's Day Good Friday & Labor day.
US traders will be in the US stockmarket as it is not a holiday. Foreign countries have their own stock exchanges independent of the US. I do not see how it will affect US stockmarket trading volume.
It's all intertwined and interdependent, due to globalization. Such an international holiday surely will negatively affect the trade volumes.
I would doubt it. If your job is to trade the US markets you will be working on your countries holiday. Keep in mind on the days the US markets are closed, you have nothing to do if it's a regular workday in your country.
Maybe the following research paper can answer: "European Trading Volumes on Cross-market Holidays" by Batrinca, Hesse, Treleaven July 2018, International Journal of Finance & Economics 23(1) https://www.researchgate.net/publication/326592330_European_Trading_Volumes_on_Cross-market_Holidays
May 1st is a labor holiday celebrated by communists and unions to celebrate workers. The US certainly didn't want to be associated with that, so they picked another date.