The Yugam, in which presently we are living, is called as "Kali Yugam". Other than this Kali Yugam, there are 3 Yugams that we have crossed and they are Kretha, Thretha and Dwapara Yugam. In all these 3 yugams, Sriman Narayanan has taken lots of Avatharams (incarnations) to protect the Dharmam against Adharmam (evil) and protect the life of his bhaktas.
Starting from the first Yugam, the Dharmam started to diminish and whenever there is a stage where it cannot survive, Sri Vishnu took the Avatars to root it. Likewise, the Avatars taken by him to root the Dharmam is the great Krishna Avatar. During this Avatar only, Sri Krishnar to Arjunan explained the great and precious "Bhagavath Geethai". This Geethai is not only explained for Arjuna, Sri Krishnar explains to this entire world, how a person should be & how he should overcome his wicked, jealous, arrogant thinking, his desire, etc. These are some of the characters that will be seen in the minds of the people in Kali Yugam. To take all the people in the Kali Yugam in the right path and motivating the bhakti, Sriman Narayanan has taken the Avathaar as Sri Srinivasar and it is 100% right that we call him as "Kaliyuga Deivam".
Sri Venkata Nivaasaya Sri Srinivasaya Mangalam".
The main sthalam of Sri Srinivasar is Tirumala (Tirupathi) and lots of social activities are happening in this great kshetram. This Tirumala has lots of great theerthams (Pushkaranis) within itself like Swami Pushkarani, Brahma theertham, etc., which are said to be so sacred. And another great and special attraction of this Tirumala is the yearly Brahmotsavam for Tirumalaiyappan.
This Brahmotsavam is done during the month of Purattasi (Sep - 15 to Oct - 15). The Perumal comes round the temple in Great Vaghanams during this Brahmotsavam. Now let us see about all these Vaghanams.
As the startup of the utsavam, first Anguraarpanam is done. And in the evening, Dwajarohanam (Kodi Yeatram) is done. Generally, Dwajarohanam is done during in the early morning time (4.30 to 6.00) but in Tirumala, it is done in the evening time.
First day:
Sesha Vaghanam:
In the night, the perumal is taken round the temple in Sesha Vaghanam along with two pirattis. This Vaghanam is also called as "Pedha Sesha Vaghanam". It is believed that this Vaghanam is the manifestation of Adiseshan.
Second day:
Hamsa Vaghanam:
This vaghanam proceeds on the second day night. The Hamasavahana signifies purity, and the Hamsa (swan) on it is known to for it is ability to differentiate between good and bad, and said to denote peace.
Third day:
Simha Vaghanam:
The perumal is taken round the temple in Simha Vaghanam in the morning of 3rd day of the Utsavam. This Vaghanam signifies the Strength and power of the Perumal (One among Sri Vishnu's Avathar)
Muthu panthal Vaghanam:
In the night of the 3rd day, the perumal comes in Muthu Panthal Vaghanam. This is a great seva for all the bhaktas.
Fourth day:
Kalpaka Viruksham:
In the morning of 4th day, the perumal coms in Kalpaka Viruksham. Kalpaka Viruksham is one among the great thing that came out from the ocean and has the tendency to give and bless us with which we ask for. The perumal who comes in this Vaghanam also has the great power and he is the one who protects us in this Kali yugam. So, this is one among the great Vaghanam during the Brahmotsavam.
Sarva Bhoopala Vaghanam:
This Vaghanam is done during the night of the fourth day.
Fifth day:
Mohini Thirukkolam:
Mohini Avathaar is one among the Avathaar taken by the perumal as a beautiful women to get the Amirtham for the Devars from the Asuras. The Perumal will be dressed very delightly during this thirukkolam and it is one among the beautiful and must see seva in the Brahmotsavam.
Garuda Vaghanam:
On the fifth day night, the perumal comes round in the great Garuda Vaghanam. During this Vaghanam, the perumal is worn with a precious jewels by named "Makara Kandigai" along with Lakshmi Haaram and lots of precious jewels. And, another speciality is during this Vaghanam, a separate Garland is worn to the perumal which comes from Sri Villiputhur Andal. This is one of the specialities of Garuda Vahanam in Tirumala.
Sixth day:
Hanumantha Vaghanam:
During the 6th day morning, the perumal comes round the temple in Hanumantha Vaghanam.
Gaja Vaghanam:
And in the evening, Vasantha Utsavam is done for the perumal along with the two pirattis. And after that, the perumal gives his seva on the Yaanai (Elephant) Vaghanam.
Seventh day:
Surya and Chandra Prabhai:
The Perumal rides on Surya prabhai (Sun chariot) on the seventh day of Brahmotsavam. On the 7th day morning, the perumal comes on the Surya Prabhai and in the night, he gives his seva on the Chandra Prabhai.
Eighth day:
Thiru Ther: (Charriot - Rathotsavam)
The eight-day of the Brahmotsavam is one among the special day and during this 8th day the perumal is seen on the ther (the chariot).
During this Utsavam, the great 10 paasurams from Periya Thirumozhi is dedicated by the great Vaishnavites infront of the Ezhumalaiyan who is found to be giving his seva in the Thriu Ther.
Guthirai (Horse) Vaghanam:
The perumal comes round the temple on the Guthirai (horse) Vaghanam on the 8yh day of the Utsavam.
Ninth day:
Theerthavari: 
Theerthavari is yet another special utsavam and has a greatness on its own. This theerthavaari is also called as "Chakrasnanam".
During this utsavam, the perumal (Chakkarathalwar is taken as the representative of the perumal) is taken towards Varaha Pushkarani and done Snanam (divine bath) for him and after this the Parayanam of the great thiruvadi mozhi is done.
Dujavarohanam:
During the 9th day night, Dwajavarohanam, (Kodi Iranguthal) is done. After this, the Parayanam of Ramanuja NootruAndhadi is done parayanam.
Tenth day:
Pushpayagam:
On the 10th day, the perumal is decorated with great flowers and the Vaghanam, pushpayagam is also very well decorated. During this utsavam,the thiruvaaradhanai is done for 12 times with those 12 different flowers which is done as the "Prayasitham" (Apologize) that if any "Kaala Niyamam" (mistakes while performing utsavam) is done.
Eleventh day:
Vidayatri:
On the 11th day, Vifsysyti Utsavam is done and this utsavam is called as"Sirama pasiharam". And during, this Vidayatri Utsavam, Thirumalayappan gives his seva towards Ananthazhwan and the Sri sadari is dedicated stating that he is being blessed by the perumal.
So, bhaktas would have now known about the Brahmotsavam's speciality and the various Vaghanams in which the perumal gives his seva. These being a short note on the utsavam, to experience this great utsavam, visit the divine sthalam, Tirumal - Tirupathi and experience the utsavam and also the beauty of the perumal in person.