I believe this reception is during the first day at sea or on Formal night (but I don't think Alaskan cruises have a formal night). As for reservations at Signatures or Latitudes, you can usually make those the first afternoon your board at around 2:30pm or so.
In Alaska there is no formal night. The Captains reception was on the second night out. As for reservations at Signatures or Latitudes you can make them on the day you board, starting at 2.30pm, outside the entrance to the Compass Rose. Personally speaking though our group (8 Europeans) didn't think much of the salmon tasting menu at Latitudes. Signatures was really good. If you want to eat there twice make a reservation for early in the cruise and then call to see if you can get a table for another night a couple of nights later!