(see High Holidays, 2nd Selecting of the Lamb)
If you missed Passover during the 14th day from sunset till sunrise of the 1st moon for certain circumstances, you can celebrate Passover during the 14th day from sunset till sunrise of the 2nd moon.
Numbers 9:6-14
AAA 6 And there were certain men,
AAB who were defiled by the dead body of a man,
A-BA that they could not keep the passover on that day:
A-BB and they came before Moses and before Aaron on that day:
AAA 7 And those men said unto him,
AAB We are defiled by the dead body of a man:
-BA wherefore are we kept back,
-B-B that we may not offer an offering of the Lord [YHWH]
-BA in his appointed season among the children of Israel?
–CA 8 And Moses said unto them,
–C-BA Stand still,
–C-BB and I will hear what the Lord [YHWH]
–C-BC will command concerning you.
–C–C 9 And the Lord [YHWH] spake unto Moses, saying,
–C–C 10 Speak unto the children of Israel, saying,
–C-BA If any man of you or of your posterity
–C-BB shall be unclean [ceremonial] by reason of a dead body,
–C-BC or be in a journey afar off,
–CA yet he shall keep the passover unto the Lord [YHWH].
—DA 11 The fourteenth day of the second month [moon]
—D-B at even [sunset] they shall keep it,
—D–C and eat it with unleavened bread and bitter herbs.
—D—D 12 They shall leave none of it unto the morning [sunrise],
—D–C nor break any bone of it:
—D-B according to all the ordinances of the passover
—DA they shall keep it.
–CA 13 But the man that is clean [ceremonial],
–C-B and is not in a journey,
–C–C and forbeareth to keep the passover,
–C-B even the same soul shall be cut off
–CA from among his people:
-BA because he brought not the offering of the Lord [YHWH]
-B-B in his appointed season,
-BA that man shall bear his sin.
AA 14 And if a stranger shall sojourn among you,
A-B and will keep the passover unto the Lord [YHWH];
A–C according to the ordinance of the passover,
A—D and according to the manner thereof,
A—D so shall he do:
A–C ye shall have one ordinance,
A-B both for the stranger,
AA and for him that was born in the land.
Reasons for celebrating 2nd Passover is not celebrating 1st Passover because you were ceremonially unclean or being in journey to keep 1st Passover and did not make it in time. Both of these valid reasons are possible signs of not keeping Torah diligently. I believe that 2nd Passover is akin to those who will receive Telestial Glory just as 2nd First Fruits goes along with the 2nd Resurrection. Keeping of a Passover shows you believe in Yeshua / Jesus as the promised Messiah, but arriving late or being defiled by a dead body, which represents sin although is not a sin, shows you are not taking YHWH’s Torah seriously.
One example of keeping 2nd Passover in the Bible is in 2 Chronicles 30.
2 Chronicles 30:2
A 2 For the king had taken counsel,
-B and his princes,
-B and all the congregation in Jerusalem,
A to keep the passover in the second month.2 Chronicles 30:15
A 15 Then they killed the passover
-B on the fourteenth day of the second month:
–C and the priests and the Levites were ashamed,
–C and sanctified themselves,
-B and brought in the burnt offerings
A into the house of the Lord [YHWH].
The 7th Seal / Wrath of the Book of Revelation the 5th Trumpet / Bowl / Vial, which is the 1st Woe, starts during 2nd Passover or 2nd Unleavened Bread which lasts 5 moons through Trumpets and possibly Tabernacles which is when Yeshua / Jesus will come out of the clouds in glory (see Second Coming for details).