This is a brief review of the laws and customs of Shavuot. Besides the status of Shavuot as a Yom Tov, there are neither biblical nor rabbinical laws relevant to it today. In biblical times Shavuot was the culmination of the Exodus, and it did not have a date of its own, but rather was marked as the 50th day after Passover. The first mention of prayers specific to Shavuot is in one of the minor tractate, Sophrim, which was composed in Israel in the eighth century and later influenced the Ashkenazi school of thought.
The first mention of the term זמן מתן תורתנו — the day in which we received the Torah — appears in the literature of Rabbi Elazr of Worms (1160-1230). The lack of specific laws of Shavuot and the renewed interest in the holiday, which started in medieval Germany, created many local customs whose purpose was to add content to the holiday. Here I will address only laws and customs not related to prayers and Torah reading.
Decorating homes and synagogues with fresh branches and flowers:
Regardless of the origin of this practice, it is a beautiful way to remind us that the Torah is part and parcel of the real world, and of the agricultural origins of the holiday and of our nation.
Like all food choices on Shabbat and Holidays (except for Matzah on Pesah), this is optional. One can eat dairy on every festive meal, or meat on every meal on Shavuot, and it will be totally fine. The mitzvah is not to cause yourself and your guests discomfort.
Staying up all night:
This is a late custom, initiated in the 16th or 17th century, following kabbalistic trends. It was originally practiced by secret societies of devout scholars, and later adopted by the broader community, especially after the use of coffee has spread through the Middle east and Europe. While it could be exciting and enjoyable for some, under certain circumstances, it should disrupt the holiday. People should know their limits and make sure that they will be alert on the day of Shavuot, so they could enjoy the holiday, pray with intention and focus, and spend quality time with their families.
Many Sephardic, and some Ashkenazi rabbis ruled that one can use electricity on Yom Tov.
Preparing for Second Yom Tov:
One can set the table and prepare food on the first Yom Tov to be able to sit early for the dinner of the second Yom Tov. It is preferable to pray and eat early, and then light candles when the first Yom Tov is over.
(You can also read more about why the time has come to keep only one day of Yom Tov, and how this goal could be achieved).