They receive instructions to preach about national holidays in their khutbahs – what should they do?

Firstly:  

There is nothing wrong with the khateeb speaking about the
battle of Badr or Uhud, or any other battle, and describing what happened on
such occasions at the time of the Prophet (peace and blessings of
Allaah be upon him). This is not regarded as celebrating it or introducing a
festival that is not prescribed in Islam, because it is simply reminding the
people of that battle and, more importantly, deriving useful lessons from
it. 

Secondly: 

With regard to national holidays, if the people celebrate
them and take them as a kind of ‘Eid, then the khateeb should not take any
part in that, and speaking about such occasions may be understood by the
listener as meaning that he is joining in and supporting this innovation. 

But if the Ministry will punish those who do not speak about
these occasions, by preventing them from delivering khutbahs or other
penalties, then the khateeb should weigh up the pros and cons, and decide
whether the interests served by his remaining as a teacher, daa’iyah and
guide will be outweighed by the negative consequences of a speech which may
damage his reputation among the common folk, or not. 

It seems that the wise khateeb can use these occasions in
order to say something useful that will reinforce the ideas of the causes of
victory and the factors that lead to defeat, the laws of Allaah with regard
to the rise and fall of societies, and the concept of al-wala’ wa’l-bara’
(loyalty towards the believers and disavowal of the disbelievers), the
necessity of turning back to Islam, and other important concepts which the
people need to know. 

In this way, the interests served by his speech will be many
times greater than the negative consequences referred to. Islam seeks to
bring and fulfil what is in people’s interests and to ward off and reduce
harmful effects, by choosing the lesser of two evils, as is well known. 

And Allaah knows best.