Short answer: were planning / arrived
had been planning is past perfect continuous tense, which means the action had been happening a while before the moment of expressing it, which is not really the case in this sentence. Also, you would not use perfect tense where the exact time -
when the news arrived - is specified, you should use simple (indefinite). We don't say
when the news had arrive because past perfect is used to express the result/effect of some action available in the past, for example
He had arrived long before we started to gather.
One more thing,
had been planning implies some long-lasting action, while
planning to do something is not such, it just means
thinking of doing something. If it was
planning the annual cultural fest celebration then it would make more sense to say
We had been planning the annual cultural fest celebration. But in any case, if the time is specified you cannot use perfect tense:
- I had gone to school yesterday.
- I have done my homework 3 minutes ago.
- You had dropped your purse when you saw me.
All of these are incorrect and should be:
- I went to school yesterday.
- I did my homework 3 minutes ago.
- You dropped your purse when you saw me.