'On vacation', 'on strike', 'on a diet': temporary statuses with 'estar de'
English describes many temporary situations with on: on vacation, on strike, on duty, on a diet. Portuguese has its own fixed pattern for them: estar de followed by a noun with no article. So you say estar de férias, estar de greve, estar de serviço and estar de dieta.
English speakers trip over this in two ways. They reach for em because it looks like the natural match for on or in, and they add an article because English often has one (on a diet). Both slips sound foreign. The good news is that the pattern is short and very frequent, so a handful of expressions will cover most of what you need.
The pattern is always the same: a form of estar, then de, then the noun on its own. There is no a or o, and the noun does not change for plural subjects. One worker or a hundred workers, it stays de greve.
The expression describes a state that will end: a holiday, a strike, a work shift, a punishment. You can often swap estar for ficar to talk about entering that state, as in ficar de castigo (to get grounded) or ficar de luto (to go into mourning).
Rule of thumb: if English says on + status and the status is temporary, try estar de + a noun without an article first.
Besides férias, greve and serviço, you will hear turno for being on shift, mostly in European Portuguese. Hospital staff in Brazil tend to say estar de plantão, and someone on leave from work está de licença. Both follow the same pattern.
Estar de viagem means to be away travelling. Estar de volta means to be back, and estar de saída means to be just about to leave.
Dieta and regime both give you on a diet. Ressaca gives you hungover, and quarentena gives you in quarantine.
A child who has done something wrong está de castigo, and a family that has lost someone está de luto.
Do not build new expressions from just any activity noun. The pattern works with a fixed set of words, so learn them as whole chunks. Some also have regional preferences. Estar de viagem is especially common in Portugal, and Brazilians often say estar viajando instead. Em quarentena is as common as de quarentena.
Posture works the same way. Estar de pé means to be standing. It is not an activity, but it follows the identical pattern without an article.
Finally, do not mix this pattern up with estar com, which is used for feelings and bodily sensations such as hunger, thirst, cold or being in a hurry. Say estou com fome, never estou de fome. Places take em, as in estar em casa.
For a temporary status that English expresses with on (on vacation, on strike, on duty, on a diet), Portuguese uses estar de + a noun with no article: de férias, de greve, de serviço, de dieta, de volta. Use ficar de for entering the state. Keep estar com for sensations like hunger and estar em for places.