Experience in the UK says that Omicron often spreads to everyone in a get-together, and comes on very fast.

They are asking people to take a home Covid test right before going to any get-together. The UK is making those tests readily available. The US seems not to want to pay for them.

Also, they are asking even vaccinated people who had contact with an infected person to take an at-home Covid test every day for the next week.

This is to slow the spread of Omicron.