New Delhi, Jan. 3 -- On New Year's Day, Bulgaria will achieve its decades-old goal of joining the euro currency union and deepening ties with the more prosperous countries of Western Europe.

Membership is expected to promote cross-border trade and investment, and the Bulgarian government pressed for years to get in. Yet polls show the changeover is taking place against a background of widespread skepticism among ordinary people.

Here are things to know as Bulgaria and its 6.4 million people become the 21st member of the European Union's shared currency:

The big switch on New Year's Day

In the run-up to the big switch, price tags and bank accounts have had to show both currencies, at the fixed rate of 51 euro cents to the outgoing curr...