In years past, holiday dinners, with generations of family members gathered around the festive table, were taken for granted. These were the times described in the Thanksgiving Day poem that begins ''Over the river and through the wood, to grandfather's house we go.''
In our mobile era, family members are often scattered as if by the wind, and grandfather's house may be hundreds of miles away. Grandmother may well be a working woman with little time for holiday feasts. This, among other signs of our times, is cited by restaurateurs as a reason for a rise of 50 percent or more in the number of families who eat out on Thanksgiving and Christmas.
But, they are quick to emphasize, tradition still lives. Ninety-nine percent of those eating Thanksgiving dinner in restaurants order turkey, and about the same number want traditional accompaniments such as mashed potatoes and gravy, creamed onions, and cranberry sauce, along with pumpkin or mince pie, for both Thanksgiving and Christmas dinners.
For Christmas dinner, meat choices vary, although turkey is still in high favor. Henry Junco, who owns four restaurants in the Albany, N.Y., area, says that ''goose is usually a sellout at Christmas, and roast duck with fig sauce is also a favorite.
''There are unusual requests on occasion,'' he continues. ''Youngsters sometimes order a hamburger and French fries, and a game hunter once asked us to prepare bear meat. We also get requests for venison.''
David Hodgkins, food and beverage manager at the Gideon Putnam Hotel in Saratoga Springs, N.Y., points out that restaurants often offer baked ham and roast beef as well as turkey for Christmas, especially if the meal is buffet style. ''Diners who have had turkey for Thanksgiving need a change,'' he says. ''Gourmet types sometimes order quail, but this should be arranged in advance.''
Mr. Junco has noticed that diners eat heavier meals on holidays, spend more money, and often ask to take leftovers home. He has also noted that fathers tend to bring their families to Thanksgiving dinner, but young people are likely to invite their parents for Christmas dinner.
The number of single people living in cities has made brunch a popular holiday meal, he adds. ''Young people alone at holiday time find brunch a happy meal, where others gather, read the newspapers, and often talk. For those on a budget, brunch is a relatively inexpensive meal, with only a nominal tip expected.''
Both Mr. Junco and Mr. Hodgkins advise holiday diners to make reservations well in advance and to be sure that the table they reserve is large enough for their party. ''The host for a large party will do well to reserve a private room , available at many restaurants,'' Mr. Hodgkins says, adding: ''If possible, arrange a special table for children. They are quickly bored by adult conversation about the holiday snowstorm 20 years ago. They will be happier and better behaved if seated by themselves.
''Be sure to check whether the restaurant of your choice is serving the holiday dinner buffet style,'' he says. ''If you have older people in the party, consider that many of them do not like standing in line for food. It is better to choose an eating place with sit-down service.''
Mr. Junco and Mr. Hodgkins both contributed to the following list of suggestions to help make a holiday meal a smooth, memorable event.
1. Make reservations for a holiday meal well in advance. Two or three weeks ahead is not too early. Be sure to include the number of people in the party; if you want a special table or a private room, this is the time to specify that.
2. Plan to eat early in the day; 2 p.m. is a good starting point. ''Employees get tired and restaurants sometimes run out of food.''
3. Arrive on time to occupy your table. Holidays are a restaurant's busiest times and some will give away a table unless the reservation schedule is honored. Most allow a 15-minute delay.
4. If your party is large, you may want to have a whole turkey brought to your table to be carved, with leftovers to be taken home. Be sure to apprise the restaurant of this in advance. If a special type of dressing is wanted, be sure to explain that.
5. For most families, the holiday meal is the focal point of the day, but don't plan to occupy a restaurant table for much more than two hours. If necessary, have your dessert served in a room that's less crowded than the main dining room, or plan to have dessert at home. ''It is not pleasant to have prospective diners watch you eat as they wait for your table,'' the restaurateurs point out.
6. If small children requiring highchairs or boosters will be in your party, let the restaurant know in advance. Most restaurants have a very short supply of highchairs.
7. If you want a dish that may not be on the regular menu, discuss it with the restaurant owner or maitre d'hotel ahead of time. Don't make it a surprise.
8. If you are ordering a whole turkey for your table or making reservations for a large group, don't be surprised if the restaurant asks for a deposit in advance. Many do that to discourage ''no shows.''
9. To please guests at holiday dinners, choose an eating place that has distinctive, attractive surroundings as well as good food. If you're on a budget , check prices in advance.