I'll be in NY next week and I would like to know if shops and major attractions opens in Memorial Day. I'm planning to go to the area of Chinatown, Soho and Southstreet Seaport for shopping or maybe Macy's and Century 21.

Department stores, major museums etc will all be open. Smaller or local stores (Chinatown, SoHo) may or may not be. You would have to check individually.

Everything in Chinatown will be open and most likely in Soho as well.

Thanksgiving, Christmas Day and New Years Day are the only time most shops close.

Pretty much everything will be open.

Most museums (Metropolitan Museum of Art, Guggenheim, etc) are closed on all Mondays, including Memorial day. They will be open the Saturday and Sunday as usual.
Memorial day is one of the biggest shopping days of the year, so you'll be with all the crowds there.

The Metropolitan Museum of Art is usually closed on Mondays, EXCEPT holiday Mondays. They will be open on May 31 (and July 5, Labor Day, etc.)

And the Guggenheim is open every Monday, incluing Memorial Day.

They are closed on Thursdays and Christmas.

I stand corrected--didn't know that the Guggenheim had switched to Thursdays off--Monday used to be sacrosanct as 'the' day for museums to be closed, all over the US.

I also forgot about holiday Mondays at the Met! They've made such a big deal about how Mondays are now often used for "high rollers" (admission to exhibitions at $50) that I forgot about the holidays .
Museums that ARE always open Mondays are the Museum of Natural History and the Museum of Modern Art (in its present Queens location as well)
Enjoy!

A word of advice about Century 21. Try to go early in the morning. I made the mistake of going mid afternoon last week and it was an absolute zoo. Very frustrating. I was told by some of their employees that it is much quieter in the early morning hours.