Utilizing Signs

 Rabbeinu Yona (Sefer Hayira) And we are not to utilize any sign except what the Sages said, and if a person has a mitzvah to do, such as review a gemora or have his son to begin to study Torah, he will not say I will wait until Rosh Hashonna or until Pesach... and those who are not concerned will never be effected, but those who are concerned the devil stands at their right to harm them with their error

Shulchan Aruch (YD 179) We have the custom to not begin things on the second or fourth days of the week (Monday or Wednesday) and we only take wives during the full moon. [Comment of Rema: And therefore, we have the custom as well to begin study on Rosh Chodesh, because even though there is no divination, there is a sign(SM"K 136). What a person knows to be inauspicious, they should not do, and they should not rely on miracles. However, one must not investigate this matter, because one should be "wholehearted" (Responsa of Ramba"n 286) as will be explained.] 

Shulchan Aruch (YD179:04) A child's or a woman's house: even if there is no divination, there is a sign. [Comment of Rema: If one has succeeded afterwards three times, or not (B"Y citing Rashi). And similarly one may say to a child, "Tell me your verse of current study" (Tur). Some say that one may make a sign about something that is still to happen, like Eliezer, Abraham's servant did, or Jonathan (Tur and R"D Kimchi) but some forbid it (Rambam and SM"G). But one who proceeds wholeheartedly and trusts in Hashem will be surrounded by kindness.]

Rabbeinu Bachya (Kad HaKemach mezuza) The common practice of the world, that they do not carry women with a partial moon but only with a full moon, there is no prohibition in this, and it is not significant, since it is nothing but simply a sign, and the intent is merely a good sign that the marriage will be complete and not lacking, and this is similar to what we practice on the night of Rosh Hashanah, and it is the custom of the Gaonim. This is also similar to the custom of anointing the kings next to a spring. as a good sign that their kingdom would last. And anyone who views the matter of marriage and the success of homes other than a sign, this is the way of the Amorites, and it is the custom of the nations who think that the success of homes depends on the star system... 

Meiri (Meishiv Nefesh 02:02) ...The simonomim of Rosh HaShanna are to increase the remembrance of G-d at every moment to avoid sin. Thus we place on the table Kara, Rubia, Kerti, Salka, Tamari, according to what is stated in Krisus anf Horious. And in order that they be viewed as simple signs and not divination, we recite appropriate words asking for forgiveness and repentance

Sefas Emes (Rosh HaShanna 5632) We have signs on the night of Rosh Hashanah, because on Rosh Hashanah, one does not ask for the needs of this world, as they said in Zohar and Tikkunim, we only ask for fear of G-d and His rule. Therefore our needs are only asked by hints