Yes evil does get stronger at certain hours of the day and certain days of the month. According to various scriptures, the demons were created by Brahma from his Tamsik body after he created night-time hence are more strong after sunset.
Vishnu Puran Chapter 5 mentions:
Brahmá then, being desirous of creating the four orders of beings, termed gods, demons, progenitors, and men, collected his mind into itself. Whilst thus concentrated, the quality of darkness pervaded his body; and thence the demons (the Asuras) were first born, issuing from his thigh. Brahmá then abandoned that form which was, composed of the rudiment of darkness, and which, being deserted by him, became night. Continuing to create, but assuming a different shape, he experienced pleasure; and thence from his mouth proceeded the gods, endowed with the quality of goodness. The form abandoned by him, became day, in which the good quality predominates; and hence by day the gods are most powerful, and by night the demons. He next adopted another person, in which the rudiment of goodness also prevailed; and thinking of himself, as the father of the world, the progenitors (the Pitris) were born from his side.
The Markandeya Puran also mentions this and also gives each category a specific time when they are the strongest:
1.8.4 Creation of the Deities
Kraustuki then asked Markandeya about the origin of the deities. Markandeya replied- 'With the objective of creating the deities, demons, ancestors and human beings, Brahma abandoned the part of his physical body in the ocean. The demons originated from the thighs of his abandoned body. Lord Brahma blessed the demons with physical bodies, which were Taamasik in nature. This part of Brahma's body became famous as Ratri (night). After that Brahma created the deities from his mouth with the help of Satva guna. The deities were given pure bodies. This part of Brahma's body which was Saatvik (pure) in nature became famous as day.'
'After this, Brahma acquired another physical body and created the ancestors. After creating the ancestors, he abandoned his body, which transformed itself in the evening. Brahmaji then acquired another body, which was affluent of Rajas qualities and thus was created human being. He again abandoned his body and from it was created Jyotsna- the transition period of day and night.'
'Jyotsna, evening and day contain the quality of Satva in themselves. Night has the quality of Taamas in itself. The deities, demons and human beings are most powerful during the day, night and Jyotsna period respectively. The ancestors are most powerful and invincible during the evening time.'
Section CLXVI of Shanti Parva of Mahabharat also mentions this association of demons with darkness:
That religion was accepted by the gods, with their preceptors, priests, the Adityas, the Vasus, the Rudras, the Sadhyas, the Maruts, the Aswins, Bhrigu, Atri, Angiras, the Siddhas, Kasyapa rich in penances, Vasishtha, Gautama, Agastya, Narada, Parvata, the Valikhilya Rishis, those other Rishis known under the names of Prabhasas, the Sikatas, the Ghritapas, the Somavayavyas, the Vaiswanaras, Marichipas, the Akrishtas, the Hansas, those born of Fire, the Vanaprasthas, and the Prasnis. All of them lived in obedience to Brahman. The foremost of the Danavas, however, setting at night the commands of the Grandsire, and yielding to wrath and covetousness, began to cause the destruction of righteousness. They were Hiranyakasipu, and Hiranyaksha, and Virochana, and Samvara, and Viprachitti, and Prahlada, and Namuchi, and Vali. These and many other Daityas and Danavas, transcending all restraints of duty and religion, sported and took delight in all kinds of wicked acts. Regarding themselves equal in point of birth with the gods, they began to challenge them and the sages of pure behaviour. They never did any good to the other creatures of the universe or showed compassion for any of them. Disregarding the three well-known means, they began to persecute and afflict all creatures by wielding only the rod of chastisement.
We come across something similar in Shrimad Bhagvatam. When Diti, one of the wives of Rishi Kashyap desirous of having children, approaches her husband at the evening time. Kashyap tells Diti to wait for sometime because the evening time was inauspicious as it was when the ghosts and goblins roamed around:
(22) With that being said, let me forthwith take care of this sexual interest of yours to beget a child. But first wait a few seconds for me to be beyond reproach. (23) This time is the least favorable moment for that, it is the awkward time at which the ghosts and their master are one's constant companion. (24) At this time of the day, oh chaste woman, at dusk, the Lord and well-wisher of the ghosts surrounding him, goes about as their king on the back of the bull [Nandî].
In the Varaha incarnation also Brahma advises Vishnu to kill the demon Hiranyaksh before night approaches:
Now Brahmaji warned Vishnu, “You’ve only an hour before the sunset. Destroy the demon before it’s dark so that he gets no opportunity to resort his black magic.” Hearing Brahma’s word’s Hiranyaksha hurled his mace towards Lord Vishnu but later flung it away. Having lost the mace, Hiranyaksha began hitting out with his fists on the chest of the boar.
Again in the Drona Parva of the Mahabharat Krishna chooses to use Ghatotkacha and his Rakshas army towards the end of the day when the dusk is close:
Then he of Dasarha's race, addressed Hidimva's son, that Rakshasa of blazing mouth and fiery eyes and body of the hue of clouds, and said these words, 'Listen, O Ghatotkacha, attend to what I say. The time is come for the display of thy prowess, and not of anybody else. Be thou the raft in this battle to the sinking Pandavas. Thou hast diverse weapons, and many kinds of Rakshasa illusion.
Ghatotkacha said, 'I am match for Karna, as also for Drona, O Bharata, or for any illustrious Kshatriya accomplished in weapons. This night I shall fight such a battle with the Suta's son as will form the subject of talk as long as the world lasts. Tonight, I will spare neither the brave nor the timid nor those that will, with joined hands, pray for quarter. Following the Rakshasa usage, I shall slay all.'
As regards specific dates in the month, the Krishna Paksha or the dark/waning phase of the month is associated more with malefic planets in astrology. As mentioned in the below excerpt:
As mentioned in the last part, it appears that from the Navmi of Krishna Paksh to the Saptami of the Shukla Paksha is a less favorable time considering the effect of all the planets including the moon. To conclude, yes the evil or malefic forces are stronger at certain times of the month and at night-time.