A merger agreement between Publicis Groupe and Omnicom Group is believed to be finalized and slated for announcement at a press conference in Paris on Sunday afternoon (8 a.m. EDT).

After days of speculation and belief in ad circles that such a massive and messy deal would never come to fruition, it appeared to be a reality after Publicis Groupe called a last-minute press conference -- at a most inconvenient time for agency executives and clients, smack-dab in the middle of a summer weekend -- describing the event as "very important" and a "major corporate announcement."

"While most industry participants and observers alike would have viewed such a combination as highly unlikely only 48 hours ago, the implications of such news can now be more fully considered, and it is profound," Brian Wieser, senior research analyst at Pivotal Research Group, wrote in a note Saturday.

Indeed, the deal would be the biggest tie-up of ad firms in history, placing the combined company, expected to be called Publicis Omnicom Group, well ahead of WPP by revenue.

The two companies on Saturday were scrambling to inform senior agency leaders about the merger announcement, but many clients were still in the dark.

Although Omnicom Group is the bigger of the two, the announcement is taking place in France, suggesting both companies' recognition of how critical it is to address Publicis' home market and clients and convince them of the deal's merit.

Whatever synergies, savings and scale can be gained, of utmost importance during the announcement, and in the days that follow, will be explaining how the deal with benefit the companies that fund Publicis and Omnicom's business, which are multinationals like PepsiCo, McDonald's, AT&T, L'Oreal and Coca-Cola.