pṛthak satreṇa vā mahyaṁ
parva-yātrā-mahotsavān
kārayed gīta-nṛtyādyair
mahārāja-vibhūtibhiḥ

Śrīmad-Bhāgavatam (11.29.11)

Either alone or in public gatherings, with singing, dancing and other exhibitions of royal opulence, one should arrange to celebrate those holy days, ceremonies and festivals set aside specially for My worship.

Gauḍīya Vaiṣṇavas observe all viṣṇu-tattva appearance days along with special fasting. Sometimes these days come next to each other or next to an Ekādaśī or Dvādaśī tithi which is meant for fasting. This happens every year somewhere in the world for Lord Varāha-dvādaśī and Lord Nityānanda-trayodaśī, but not at all locations due to different sunrise times.

The way Śrīla Bhaktivedānta Nārāyaṇa Mahārāja recommends observing these Vaiṣṇava festivals, for example, Ekādaśī, Śrī Varāha-dvādaśī, and Śrī Nityānanda-trayodaśī is as follows:

In countries or cities where the first day is Ekādaśī which is suitable for fasting, the second day is the appearance of Lord Varāhadeva, and the third day is the appearance day of Lord Nityānanda (Nityānanda-trayodaśī): 

• Those who follow nirjala Ekādaśī (full fasting, even from water) may break fast with a drop of caraṇāmṛta  after the worship of Lord Varāhadeva and continue fasting on Lord Varaha-dvādaśī if desired (depending on health) or
• Those who follow anukalpa (drinking water or respecting Ekādaśī preparations) on Ekādaśī tithi on the next day should first offer ārati to Lord Varāhadeva, pray for forgiveness* [see endnote 1], and then break the fast** [see endnote 2] with grains and continue to respect regular prasādam (grains and all other offerings).
• Then, on the third day – Śrī Nityānanda-trayodaśī – devotees should fast until noon, then take Ekādaśī preparations observing the Ekādaśī rules and regulations. (Sometimes, but very rare and not recommended, a Vaiṣṇava may  do nirjala fasting on all three days breaking fast the second two days with a drop of caraṇāmṛta and with grains the day after Śrī Nityānanda-trayodaśī.)

In countries or cities where Ekādaśī fasting is observed on Dvādaśī tithi:

• The devotees who follow nirjala Ekādaśī fasting (on Dvādaśī), can break their Ekādaśī fast by honouring caraṇāmṛta on the next day (Śrī Nityānanda-trayodaśī), and for the rest of the day respect anukalpa (regular Ekādaśī preparations) or continue fasting. In this situation, grains should be taken the day after Śrī Nityānanda-trayodaśī to break their fast completely at the appropriate time.
• For those devotees who observe Ekādaśī fasting (on Dvādaśī) respecting offerings of anukalpa (water or regular Ekādaśī preparations), the next day they should first offer ārati to Lord Nityānanda, then hear His glories. After this they should pray for forgiveness* [see endnote 1] and then they may break their fast by taking grains. The rest of the day they may respect regular prasādam.

*Endnote 1 – generally the appearance day of Lord Varāhadeva, as with any incarnation’s appearance day, is observed by taking only Ekādaśī preparations.

**Endnote 2 – to know the exact time of breaking the fast for your city, please check the Vaiṣṇava calendar.

Source: Purebhakti.com

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Unless indicated differently, all verse translations and quotes are from the books by Śrīla Prabhupāda (Vedabase.com)