If you are with a customer and he orders one then OK. If you are with the boss then no.

We all advocate the reading of the minutes and then a short meeting.

All the luncheon was a success, especially when he advocated the free punch.

"Teacher's Luncheon" means that only one teacher could attend. "Teachers' Luncheon" denotes the plural form, i.e. a luncheon for many teachers.

Luncheon is a noun.

When lunch is served to several people it can be called a luncheon.