be on duty 

值班

(zhíbān )

A: I'm afraid that I can't attend next month's gathering.

我恐怕参加不了下个月的聚会了。

(wǒ kǒnɡpà cānjiā bù liǎo xiàɡèyuè de jùhuì le.)

B: What happened? That is the only party that we have over recent years. 

怎么了？那可是我们几年来唯一的聚会。

(zěnme le？nà kěshì wǒmen jǐnián lái wéiyī de jùhuì.)

A: Sorry, I need to be on duty that day. 

对不起,那天我要值班。 

(duìbùqǐ,nàtiān wǒ yào zhíbān.)