Counting 14 nights forward from Opata Roa (17 or 4 + 17 = 21) we will arrive at Rano Raraku - given that item 28 is counted for 2:

Rakau (the Tree)
17 Opata roa a mana aia.
18 vai tara kai u(a) a ngao roaroa a ngao tokotokoa.
19 hia uka a hakairiiri a hakaturuturu.
20 hanga ohiro a pakipaki renga.
Waning Moon
21 ko roto kahi a touo renga.
22 ko papa kahi a roro.
23 ko puna a tuki a hauhau renga
24 ko ehu ko mahatua a piki rangi a hakakihikihi mahina
25 ko maunga teatea a pua katiki.
26 ko te hakarava a hakanohonoho.
New Moon
27 ko hanga nui a te papa tata ika.
28 ko tongariki a henga eha tunu kioe hakaputiti.ai
  ka haka punenenene henua mo opoopo o tau kioe
29 ko te rano a raraku.

The last 4 lines probably are to be counted as the first of next month, which means that number 26 (or with 4 added - the 30th night) is the last night of the current month.

Hanga Nui evidently is the first night of the new month. Opata Roa is the first night beyond Waxing Moon and Hango Ohiro is the last night to be counted.

Waning Moon is not 14 nights long, only 6 nights long.

4

New Moon

4

The Tree

16

Waxing Moon

 6

Waning Moon

20

nights to be counted

10

nights not counted

Maybe they crossed over items 30-31 in order to underline that a month should end in another way than the missionaries' alternating 30 and 31 days:

30 ko oparingi a a uuri
31 ko motu kumu koka a kaoa