This is not specific to poly - blended families, families with partnered adult siblings. Be flexible and realize that you are not obligated to attend every event you are invited to. For instance, I don't go to weddings, funerals, baby showers, bridal showers etc., for anyone other than my immediate family. My family is incredibly flexible, so we just always plan to do our get-togethers the Saturday before or after the holiday, so people are free to do the holiday with their in-laws, by themselves, or not at all. We celebrate all of the spring birthdays when we get together around Easter and the fall birthdays around Thanksgiving.
I think similar sorts of general flexibility can work for poly peeps too. Kids' stuff comes first, if applicable, then adults can request the holidays or days that are meaningful for them personally. Easter is my least favorite holiday. I don't do Valentine's day, my birthday or anniversaries. I like 4th of July! And the School Bus Demolition Derby at the County Fair the 3rd Wednesday in August!
JaneQ