What an Esbat is
An Esbat is a moon observance, most often at the full moon, as distinct from a Sabbat, which marks the seasonal Wheel. In coven practice Esbats are the working meetings: magic, teaching, business and celebration.
The word
The term reached modern Wicca largely via Margaret Murray's writing in the 1920s and 1930s, where it was used for supposed regular witch meetings. Murray's wider witch-cult thesis is rejected by historians, but the vocabulary stuck.
Solitary Esbats
Alone, an Esbat can be as small as ten minutes at a window: notice the moon, say what the month has been, name one thing to grow and one to let go, write it down. Some solitaries mark every full moon; many mark whichever ones they happen to notice.
Not every full moon needs an event
Thirteen elaborate rituals a year is a lot of evenings. Frequency is a practical decision, not a measure of devotion.