Ethnographic data collected on Easter Island in the late XIX and first half of the XX century suggest that the extant rongorongo tablets contain songs, legends or other chanted traditions. However, we have yet to succeed in relating any one of the rongorongo texts to one of the many legends collected by ethnographers. An interesting observation is that, while none of the Rapanuis with whom early visitors to the island were acquainted mentioned anything about astronomy in the context of rongorongo tablets, the only piece of rongorongo texts whose meaning we are certain of is the “calendar tablet Mamari”. In the four lines of this tablet, also known as rongorongo text C, we encounter 30 moon glyphs arranged in a pattern that mirrors the Rapa Nui lunar calendar as recorded by early Western visitors. This presentation argues that yet another rongorongo item – tablet Keiti, also known as text E – contains a pattern that can well be interpreted as another calendrical instruction, quite similar in nature to the Mamari calendar. Engraved on tablet Keiti is a string of glyphs known as sequence alpha 1-10, which is repeated 10 times throughout one side of the tablet. This repetitious string consists of 9 to 11 conserved glyphs, among which we find two moon glyphs. Moon glyphs can be engraved in two forms: facing right (encoded as 040) and facing left (041). In Keiti's alpha sequence we encounter right and left facing moon glyphs at seemingly random intervals. However, closer examination of moon glyphs in the alpha sequence shows that they are arranged in a pattern that can be interpreted as a list of ten months, whose length varies between 29 and 30 days, thus approximating the natural length of 29,5 days per month.