don't know about weaker but there's a veil between our world and theirs which grows thicker and thinner throughout the year. [thinnest at Samhain and Beltaine] so at certain points communication is easier or harder depending on the veil. i've dealt with spirits my whole life and i've noticed they have various energy patterns, it depends on the spirit and the veil. [example, one was most active in september and dormant the rest of the year, while another was active every 4 months. a third always had the same level of energy because she didn't do much]
spirits don't typically harm people, but their energy levels help them do stuff [like communicate with a pendulum for example] the thinner the veil the less energy is required from the spirit [which is why so many try spirit work around Samhain/Halloween] spirits also use energy to move objects and manifest, so it would require a ton of energy for a spirit to say, fully appear before you and throw a mug at your face. this is one of many theories why spirits don't manifest and talk to people, some have less energy than others [again, i've had a few spirits who never spoke or manifested, but you could sense them, while others you could hear voices or objects would randomly disappear and reappear.] personally i've never been attacked by a spirit but i've had spirits grab me or poke me even slap me once [long story] but it wasn't anything dangerous [i had a long friendship with a spirit and sometimes he would poke me or whatever as a joke]
yes, solstices are important to many witches. [not all, it depends on the path] the winter solstice is Yule and is very similar to Christmas. it is the longest night of the year and is beleived to be the rebirth of the sun/God [Wiccan view anyway] there's also Ostara, Litha, and Mabon all celebrated by various witches in various ways. Litha is the first day of summer, the God is at his most powerful, but soon will die. we celebrate the warmth and the crops. if you follow the Oak and Holly Kings, this is the day the Oak king is defeated by the Holly King who will bring winter. [in some Wiccan circles Litha marks the day the Goddess begins to rule the year and the nights grow longer because she represents the moon] it's a time to be spent outdoors celebrating, but it has this strange gloomy undertone to it in my opinion. each soltice does have a story, energy, ritual, and even spell casting that's benificial at that time. Litha [or Midsummer] is a time where people try to connect with faeries. personally i go for a picnic with my family in the park.