Formal invitations follow a specific set of style rules that informal invitations don't need to follow, even when indicating the time. When telling your guests what time to arrive and how long they may stay, you'll show the time in an elegant manner that highlights the formality of the event.
Writing the Time Out
While using "6 p.m." is acceptable on informal invitations, formal invitations should have the time fully written out with the phrase "o'clock" included. For example, an invitation might request your presence "at six o'clock in the evening." While the hosts are not obligated to specify whether the event takes place in the morning or evening, it might be necessary to indicate the time of day in certain cases, such as when the event might start at 8 or 9 -- times that could very easily be construed as morning or evening. Don't use "a.m." or "p.m." abbreviations.
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Quarter and Half-Hours
If the event falls on the half-hour, a formal invitation phrases it, "... at half after six o'clock." The word "after" is more appropriate here than the word "past." If the event falls on the quarter-hour, the appropriate wording is, "One quarter after six o'clock" or "three quarters after six o'clock," with no hyphen between "three" and "quarters." Write "twelve o'clock" rather than "noon" or "midnight," though you may indicate that the event starts in the afternoon along with the time.
Morning, Evening and Afternoon
When specifying that an event takes place in the morning, evening or afternoon, that your clock time correctly corresponds with the right time of day. Traditionally, morning includes all hours between midnight and 11:59 a.m.. Afternoon begins at noon and lasts until 5:59 p.m., while evening starts at 6 p.m. and ends at 11:59 p.m.. Don't make the mistake of indicating that 5 p.m. is in the evening because it's really in the afternoon.
Spacing and Line Breaks
Typically the opening line of an invitation lists the name of the host, followed by a line that formally invites the person being addressed. The next line, if it is a wedding invitation, indicates who is getting married. Directly below that is the day of the week and the day of the event. The time of day is generally indicated on the last line. For example:
Mr. and Mrs. Henry Dashwood
cordially request the honor of your presence
at the wedding of their daughter, Elinor Dashwood
Mr. George Knightley
on Saturday, the tenth of October
at five o'clock in the afternoon.