This week Friday is Shavuos. Can you and maybe should you make early Shabbos?
There is absolutley no problem with making early Shabbos. In fact, the Mishna Berura in Siman 527 Sif Katan 3 writes that the minhag was to make early Shabbos specifically on this Friday. The reason is as follows.
The Gemara in Pesachim (46a) has a machlokes Rabba and R' Chisda, is there an issur d'oraysa if you cook on Yom Tov for a weekday. Rabba says no and R' Chisda says yes. Rabba says no because of ho'eel, since guests can come and eat the food it falls under the heter of ochel nefesh. R' Chisda disagrees. Rabba asks R' Chisda according to you how do we cook on Yom Tov for Shabbos? R' Chisda answers that tzorchei shabbos naasim, b'ytom tov, that you are allowed to prepare from Shabbos to Yom Tov.
There is a big nafka mina between the 2 opinions. What happens if you finish cooking very late in the day right before Shabbos? According to Rabba you are in trouble, the heter of ho'eel does not apply as there was not time for guests to come and eat the food on Yom Tov while according to R' Chisda there is no problem as you are allowed to prepare from Yom Tov to Shabbos.
It is not clear who we pasken like, Rabba or R' Chisda. Based on this the Mishna Berura writes that we should be choshesh for Rabba's opinion and finish cooking early in the day. Therefore the Mishna Berura writes the minhag was to make early shabbos.
The question is how does the minhag help. There are 2 possibilities:
1. It is a pyschological thing. If you make early shabbos then you will prepare earlier in the day.
2. It is a din.
There is a a big nafka mina, many people will finish their preparation 2 minutes before candle lighting whenever it is, therefore if early shabbos is because of 1 you gain nothing as people will finish cooking 2 minutes before lighting candles for early shabbos.
What is pshat that it is a din? The answer may be as follows. When you make early shabbos, you are turning a day of chol into Shabbos, you are adding kedusha. However, when that day is also Yom Tov, you cannot take away the kedushas Yom Tov, you can only add kedusha. Therefore, that time period of early shabbos has both kedushas. If this is true, if you finish cooking before early shabbos, then there is still time for ho'eel to kick in, until Yom Tov is actually over. Any food eaten before shkia will be a kiyum of simchas Yom Tov as well and therefore the heter of ochel nefesh would apply as well as the sevara of ho'eel.
To sum up, this Friday you should try to finish any cooking early in the day to be yotze Rabba's opinion and it is a good idea to make early Shabbos.