Það er algengt að ég vakni of snemma á mánudögum, en svona rólegur morgunn er sjaldgæfur.

This is a very common Icelandic pattern meaning It is common that ... or more naturally in English, It’s common for ... / It’s common that ....

• það = it
• er = is
• algengt = common
• að = that / for

So Það er algengt að ég vakni ... means something like It is common for me to wake up ...

Notice that algengt is neuter singular here because it matches the impersonal það er ... structure.

Vakni is the subjunctive form of vakna.

In Icelandic, after expressions like það er algengt að ..., it is very common to use the subjunctive in the subordinate clause.

So:

• ég vakna = I wake up or I am waking up in the indicative
• að ég vakni = that I wake up in the subjunctive

The subjunctive often appears after clauses that express:

• opinions
• possibility
• evaluation
• general tendency
• uncertainty

Here, Það er algengt að ... is describing a general tendency, so vakni fits naturally.

Of snemma means too early.

• of = too
• snemma = early

A useful point: snemma is an adverb, not an adjective, because it describes when the waking happens.

Compare:

• snemma = early as an adverb
• snemmur / snemma / snemmt would not be used here in the same way

So ég vakni of snemma = I wake up too early.

Á mánudögum means on Mondays in the sense of a repeated habit.

• á = on
• mánudögum = dative plural of mánudagur (Monday)

This plural form shows that the speaker means Mondays in general, not one specific Monday.

So:

• á mánudögum = on Mondays
• á mánudaginn = on Monday / this Monday in a more specific sense

This is a very useful distinction in Icelandic time expressions.

Because the preposition á often takes the dative when it refers to location or time rather than motion.

Here it is a time expression, so we get:

• á mánudögum = on Mondays

Also, because the meaning is plural and habitual, Monday appears in the plural dative form.

So the grammar is:

• preposition á
• time meaning
• therefore dative
• repeated Mondays
• therefore plural

Svona here means such, this kind of, or like this.

In the phrase svona rólegur morgunn, it suggests:

• such a quiet morning
• a morning like this
• this kind of quiet morning

It points to the type or quality of the morning.

So the second clause means something like:

but a quiet morning like this is rare

Because rólegur must agree with morgunn in gender, number, and case.

Morgunn is:

• masculine
• singular
• nominative here

So the adjective must also be:

• masculine
• singular
• nominative

That gives:

• rólegur morgunn = a quiet morning

If the noun were neuter or feminine, the adjective form would change.

Sjaldgæfur means rare.

It ends in -ur because it also agrees with morgunn, which is masculine singular nominative.

So both adjectives in the second clause match morgunn:

• svona rólegur morgunn
• er sjaldgæfur

Literally, that part is:

• such quiet morning
• is rare

More natural English: such a quiet morning is rare

Because Icelandic does not have an independent indefinite article like English a/an.

So where English says:

• a quiet morning

Icelandic simply says:

• rólegur morgunn

If Icelandic wants to say the quiet morning, it usually adds the definite article as a suffix to the noun instead.

So:

• morgunn = a morning / morning
• morguninn = the morning

This is normal Icelandic word order for a simple statement:

• svona rólegur morgunn = subject
• er = verb
• sjaldgæfur = predicate adjective

So the structure is:

subject + verb + complement

The conjunction en means but, connecting the two clauses:

• Það er algengt að ég vakni of snemma á mánudögum
• en svona rólegur morgunn er sjaldgæfur

So the whole sentence contrasts two ideas:

• waking too early on Mondays is common
• a morning like this is rare

Yes. They form a very clear contrast in meaning.

• algengt = common
• sjaldgæfur = rare

That contrast is one of the main ideas of the sentence:

• waking up too early on Mondays is common
• having a calm morning like this is rare

This makes the sentence feel very natural and expressive.

Yes, but the meaning would change.

• á mánudögum = on Mondays, as a habit or recurring event
• a singular form would usually refer to one specific Monday

So the plural is important here because the speaker is talking about something that happens regularly, not just once.

That is why á mánudögum is the best choice in this sentence.