I understand that Sunday, May 4th, and Monday, May 5th, are Remembrance and Liberation Day respectively. Is there anything I should plan to do or plan not to do on those days? Thanks.

In what country?

Nov 11 is Remembrance day in the UK and France and elsewhere, also called Armistice day.

May 8 is V-E day.

May 1 is Labor day.

May 4th is rememberence day in the Netherlands, May liberation day.
Rememberence Day is in the evening. Every city town and village has it's on memorial and service, with two minute silence at 8pm. Flags fly ay half mast from 6pm until 9.10pm. The silence is respected througout the Netherlands. Shops close early that day.
The main rememberence is at the monument on the Dam in Amsterdam, which is attended by the Royal Family, so security is tight. Scheveningen is also an important place for the remberence day events.
Liberation Day is a free day for civil servants, and in years ending in 5 or 0 for the rest of us (unless it is in the weekend when iit is just a normal weekend day - no day off in lieu). There are a few festivals held around the country to mark it, but as it is a normal working day it goes by unmarked by most.

Oh and don't forget the May 1st is Ascension Day so also a holiday in NL. We don't celebrate Labour day.
Plus of course April 30th is Koninginnedag.

What is likely to be closed on Ascension Day?

We still need to knpw what country you are asking about. Is it the Netherlands? That seems to be the only place celebrating anything other than Labor Day.

The Netherlands.

Pretty much all the shops will be closed on Ascension day (furniture stores and garden centres tend to be open), but most museums and tourist things will be open.
In my town we have an anuual market (de Oude Ambachtenmarkt)demonstrating typical old crafts, and selling the products. They also have things like sheep shearing going on as well. No doubt there are other similar things around the country on Ascension Day.

I was in Paris last VE Day- there was a neat ceremony at the Arc de Triomphe, but not much else different.

Sorry for resurrecting this thread, but hetismij noted that Scheveningen is an important place for the Remembrance Day events. Why is that, and would it be a good day to visit that city. Thank you.