The level of formality of the invitation should accord with that of the occasion.
Formal Private Functions
There are two kinds of invitations to formal functions which, unless time is short, are prepared on cards engraved in script from a copperplate:
-Formal occasions, such as luncheon and dinner parties
-'at Home' invitations for all other parties, such as receptions, garden parties, luncheons, dinners and suppers. (On the invitation the ‘a’ for ‘at’ is lower case and the ‘H’ for ‘home’ is upper case.)
Informal Private Functions
Invitations to informal functions may be extended by letter, telephone or email.
It is up to the hosts to decide, and make clear to guests, the appropriate level of formality and all the other relevant details and arrangements.