In general, eggnog is not available year-round because demand for the product drops dramatically in the summer months, and production is not cost-effective. Eggnog flavoring mix is available year-round on Amazon.com as of 2015.
Supermarket sales of eggnog usually begin picking up in mid to late October. Sales peak around Christmas, then drop off sharply at the beginning of January. Most dairy companies stop production of eggnog at this point in the year because they are no longer able to profit from the sales.
Aspen Mulling eggnog mix is a powdered mix available year-round that consumers may add to milk or a milk substitute to produce a drink with a similar taste to eggnog.