Is dinner a countable?

She is having (A?) dinner with her family.

The dictionary says dinner is a countable tho...
But should I put the A or not?
Why or why not?

THANK YOU : )

"have a dinner" normally means that it's a special occasion planned to celebrate or mark something. Daily use is "have dinner with".

M56,

I'm curious. Where did you pick that knowledge up? Did you learn it in a class, or did you pick it up in reading/listening? I'd be very interested to hear. ^_^ Thanks in advance.

it both sounds good to me..

<I'm curious. Where did you pick that knowledge up? Did you learn it in a class, or did you pick it up in reading/listening? I'd be very interested to hear. ^_^ Thanks in advance. >

I've used it all my native-speaking life. It's fairly common in BrEng formal register:

1 A1W evening, as Patron, The British Nutrition Foundation, attends a dinner at the Natural History Museum, South Kensington, London SW7.
2 A29 Highness, Patron of the British Nutrition Foundation, attended a Dinner at the Natural History Museum (Chairman of the Council of the
3 A3V visited Stirling Castle, Strathallan Games Park and subsequently attended a dinner at the Forum Hotel, Glasgow. His Royal Highness, attended
4 A47 , as President, Engineers for Disaster Relief, attends a Dinner at the Institution of Civil Engineers, Great George Street, London
5 A4U from the Leader. This occurred on Monday night at a dinner at the Imperial Hotel. When she sat down we rose in
6 AD2 relations with representatives of the Republic's government by attending a dinner at which Garret Fitzgerald, the Republic's Prime Minister, was
7 AHU the tradition each year for the reigning champion to host a dinner at the Augusta National Grill Room for all the other champions,
8 ALV of the RSC was celebrated by the Downland section with a dinner at the Gatwick Manor hotel in October. About 80 representatives of
9 B0H in Barbara Castle's diary for 3 January 1968 records a dinner at Crossman's Westminster home where she, her host, Peter
10 BPK The rest of the money was used to pay for a dinner at the Crescent Hotel. These dinners became an important part of
11 CBB Stamford in May 1855. She had fallen ill following a dinner at her son's house and had shown symptoms of arsenic poisoning
12 CHW share in a unique treble when the Lancashire RFU held a dinner at Old Trafford to celebrate their senior team winning the Senior County
13 EC5 School at Athens. The highlight of the trip was a dinner at the British Embassy, hosted by HE the Ambassador (Sir
14 ED9 the play. The Gala Performance will be followed by a dinner at the Savoy. Theatre tickets £115 including VAT, the best
15 EDA his superiors of the service he had performed. After a dinner at the famous Kaiserhof Hotel, chosen because Hitler had made it
16 F9U On one occasion Minton, too, was invited to a dinner at which a famous pianist was present. Everyone, Minton above
17 FNX Six weeks before the baby was due, Branson attended a dinner at the Venue. It was a long evening, and he
18 GXA Grouping and Chairman of the Glasgow Football Club, hosted a dinner at Glasgow's Central Hotel. Prizes were presented by Royal Bank
19 HAB Finally, I notice that the Scottish Division is holding a Dinner at the Caledonian Hotel in Edinburgh on November 7th. The fact
20 HY5 the Prince of Wales himself had to be excluded from a dinner at which the Russian ambassador was to be present, since it
21 K4T Hennessy Player of the Year. Pears was honoured at a dinner at Newcastle's Civic Centre last night. Hail the conquering heroes
22 K57 from Bill Cotton, president of the RTS, at a dinner at the London Hilton hosted by fellow BBC journalist Moira Stuart.
23 K5M Some of the corporation's governors were expected to attend a dinner at Broadcasting House last night, with the Birt issue the main
.....

1 CEN Monday. The winners will be announced later this month at a dinner hosted by David Frost at London's Royal Albert Hall. Gazumping
2 HF0 a couple of nights ago she was in Birmingham at a dinner hosted by the Birmingham Post. There is of course nothing new in
3 HKP had objected to his non-notification of and subsequent exclusion from a dinner hosted by Menem on Jan. 17, when one of the guests
4 HKV tunnel or bridge link between the two countries. At a dinner hosted by King Juan Carlos, King Hassan avoided mention of Morocco
5 K2N Sinn Fein president Gerry Adams, and met Unionist members at a dinner hosted by the American consul-general. He welcomed the European Commission on
6 K5M 's most highly acclaimed broadcasters are planning a boycott of a dinner hosted by Mr Hussey tomorrow night for Sir Michael Checkland, Mr
....

If you need more example, just ask.

Bennyfishy...

2 [C usually singular] a formal social occasion in the evening at which a meal is served:
They held a dinner to celebrate his retirement.
a charity/society dinner

Time mag:

"woman, who called Cosby a onetime friend and mentor, said she took three pills he gave her and awoke to find her bra undone and her clothes in disarray. Cosby's lawyers said that after a dinner with the woman, he gave her Benadryl when she complained of stress and sleeping problems."

Edited typo.

<<If you need more examples, just ask. >>

Thanks a lot for all of your answers!
But I was a bit confused reading the line from Time Magazing provided by Divvy..where "a dinner" is just so fine...

Why is it?

<But I was a bit confused reading the line from Time Magazing provided by Divvy..where "a dinner" is just so fine...>

Maybe it was a special occasion.